In the first string of condition: "For example, 2^2−1 — the first Mersenne prime".
So, the first Mersenne prime is 2^2-1 = 3.

Then, in the sample we read:
18   -   3217

But, if so, the first Mersenne prime is 2, not 3, so, for the test

1
1

the program should output 2, not 3.