I was recently wondering what complexity loop can be used in less than 1 sec. For the same, I took one of my previously solved problems and added an extra loop but, even after I increase the looping to as high as 10^15, I get Accepted within 15ms. How could this happen?

Problem link: http://mirror.codeforces.com/problemset/problem/371/C

Solution link: http://mirror.codeforces.com/contest/371/submission/27344927

PS: If this a bad place to ask for this query, please let me know where to post this. :)