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

Ex Input:
Problem: what is the greatest prime factor of 2 ^ 10 - 1 ?

Ex Output:
3

Ex Input:
Problem: what is the units digit of 17 ^ 83 13 ^ 82 11 ^ 67 ?

Ex Output:
4

Ex Input:
Problem: find the sum of divisors of 544 which are perfect squares .

Ex Output:
21