The task is to generate the correct answer for a given math problem.

Ex Input:
Problem: a man is 28 years older than his son . in two years , his age will be twice the age of his son . the present age of the son is ?

Ex Output:
26


Ex Input:
Problem: find the least number must be subtracted from 105829 so that remaining no . is divisible by 21 ?

Ex Output:
10


Ex Input:
Problem: what least number must be subtracted from 42739 so that remaining no . is divisible by 15

Ex Output:

4