Sven-Goran Eriksson is a Sudoku devotee and he knows the slightest mistake can ruin everything.
Eriksson scribbles away at the Japanese number puzzles to pass the time on long journeys between watching football matches around the world.
The Swede's mental dexterity and patience in the pursuit of problem solving has helped him though his England career.
A national team manager can sit for days, waiting to see his players, turning possible solutions over in his brain and fine-tuning preparations.
Eriksson has proved calm, thoughtful and thorough in most professional aspects of his job.
He will leave the role after the World Cup finals and his egg-head image as one of the modern game's thinkers is largely intact.
He arrived in England, telling the world how he liked classical music and Buddhist poetry, in stark contrast to his predecessor Kevin Keegan.
But not everyone has been convinced by the ice-cool sophisticate from Scandinavia.
When England crashed out of the 2002 World Cup in the quarter-finals to Brazil, Eriksson came in for as much flak as blundering 'keeper David Seaman.
The manager did not have the tactical know-how to change the game, critics claimed, and his touchline antics lacked the passion to inspire his players.
At that moment he was dubbed a quarter-final coach and it has stuck.
He can organise his vastly superior players to top their qualifying groups and group games when the seeding system separates England from the world's top teams.
But stick him up against Brazil, France or Portugal on home turf and the same team lacks the X-factor.
Eriksson's failure to perform in the touchline technical area zoomed back into the spotlight as his team were humbled in Northern Ireland last year.
Coach Steve McClaren was the man flying around yelling at highly-paid and under-performing Barclays Premiership stars who were beaten 1-0 by a team drawn mainly from the Coca-Cola Leagues.
That night in Belfast will forever be Eriksson's low point as England boss. It sparked an angry reaction in the papers and fans chanted "Sack the Swede".
But he took the hits and came back, qualifying automatically for Germany as he always insisted he would, and then lifting hopes again with a thrilling win over Argentina in a friendly.
Eriksson has proved his resilience. On the pitch, moments like Northern Ireland have been few and far between.
He has been criticised for devaluing England caps but his competitive defeats have usually been at the hands of good opposition in typically gallant circumstances.
Off the pitch, however, he has lurched from one embarrassing situation to another and it was no great surprise when the scandal of the fake sheikh called time on his tenure as England boss.
A reporter, posing as an Arab millionaire, extracted a glut of sensitive opinions from Eriksson and it was time for the FA to act.
He had survived liaisons with Ulrika Jonsson, Manchester United, Roman Abramovich and Fariah Alam but this would be the last time he embarrassed his bosses inside Soho Square.
A series of meetings ended with FA chief executive Brian Barwick telling the Swede he should make this season his last.
"I haven't taken any decisions. I've literally done what people told me to do," said Sven, when asked whether his situation was irreversible.
That was a yes. There's no way back for him after his comments about the English game, some of his key players and admitting an interest in leaving England for Aston Villa.
Perhaps it was always going to end a bit like this. Eriksson has never understood why the private life of a football manager would be of interest to the general public.
He points only to his record in competitive football, qualifying for three tournaments out of three.
That's the consistency you get for £4million a year, is his argument. But he has been lucky enough to work with England's best generation of talent in 35 years.
Eriksson's public support will always remain high on the back of one night in Munich, in September 2001.
England looked unlikely to qualify for the 2002 World Cup finals until Eriksson swept into the job from Lazio.
The team had been beaten at Wembley by Germany, prompting Keegan's resignation, and then drew in Finland under the temporary leadership of Howard Wilkinson.
Eriksson transformed the campaign thanks to the 5-1 win over Germany and eventually won the group.
That victory remains his finest hour - and only a World Cup win in July will eclipse it - but he alienated some supporters with his approach to friendly games.
His fetish for using six or more substitutes soon turned people off although Wayne Rooney was given his first cap in an otherwise forgettable defeat to Australia, at Upton Park, in 2003.
It was a game when Paul Robinson also made his debut, offering Eriksson some support in his argument for experimentation, as the Spurs 'keeper is now his number one.
Eriksson, however, can still shape history and demand that England's first era under a foreign manager will go as a success.
The 23-man squad he takes to Germany on June 5 should be the most-gifted and best-equipped England squad ever to travel overseas to a major tournament.
Assuming none of his key players is hit by injuries, Eriksson's squad contains genuine world-class players, with the right balance of youth and experience.
The finals are in Germany, with no extreme climate conditions and no time-zone problems and Group B should not present anything to stop England reaching the knockout stages.
Eriksson also has his players together for four weeks before the first game against Paraguay on June 10.
There are no excuses this time and, to be fair, he is not searching for them.
Eriksson, in five years at the helm, has neither put the country off foreign coaches completely nor made an indisputable case for them.
He has done well but not that well and has one wonderful chance to crack the puzzle of the World Cup and go out in glory.
It would leave a nation to regret ever sacking him.
Eriksson has been chased out of the job he loves by a man in fancy dress but if he gets his hands on the World Cup the joke will be on England.