'80s in Poland. Math professor at Warsaw university finds out he would earn more money if he worked in a shipyard, so he quits his job and find a job in the doc

'80s in Poland. Math professor at Warsaw university finds out he would earn more money if he worked in a shipyard, so he quits his job and find a job in the docks. And indeed he earns more money. One day he overhears that there is an evening school and every worker who will attend will receive 10% bonus, so he signs up for it. First day in this evening school, very first class is math. But they have such a trivial topics that professor became bored and reads newspaper under the table. Teacher notices it and call him up to calculate the area of a circle. Professor is so surprised that he suddenly can't remember the formula, so he converts it in his head to polar coordinates and uses integral to calculate the area, and then write the formula on the table: S = -PI * r * r Then he scratches his head and start thinking where the minus came from when suddenly someone shouts from the back row: "flip the interval of integration, colleague!".