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

Ex Input:
Problem: a sum of money at simple interest amounts to rs . 815 in 3 years and to rs . 878 in 4 years . the sum is :

Ex Output:
626


Ex Input:
Problem: if a - b = 5 and a 2 + b 2 = 31 , find the value of ab .

Ex Output:
3


Ex Input:
Problem: a garrison of 2000 men has provisions for 54 days . at the end of 18 days , a reinforcement arrives , and it is now found that the provisions will last only for 20 days more . what is the reinforcement ?

Ex Output:

1600