The mantissa of 290990con is 0 before the letter. Mantissa is an abacus term, which refers to the last digit of a number, which can also be called "precision", that is, a number that has an important influence on the precision of the expression result. Mathematically, it is specially used to refer to the number after the decimal point, and in real life, it also refers to the last few digits of the number such as bookkeeping.