I found a very strange problem in Educational Codeforces Round 50 F. Relatively Prime Powers.
First, here is a AC code written by me. https://codeforces.com/contest/1036/submission/42680029
And If you add a line "assert(val != 999999999999999956LL);", then this code will fail on test 3. It means test 3 has a case that num = 999999999999999956. https://codeforces.com/contest/1036/submission/42680073
But This is another AC code written by @alonefight, https://codeforces.com/contest/1036/submission/42657108. I ran it in my local environment and compared with my code. The result is surprised.
I used the test case as follow. 1 999999999999999956
My result is 999999998998996625 But @alonefight's result is 999999998998996624
I submitted his code and it still passed all test case. https://codeforces.com/contest/1036/submission/42680102
FYI, my environment is macbook, clion, c++14.
Can anyone tell me why this strange thing happened?