When I wrote solution of this problem in C, I have got WA#3. Main idea - number (K*8-7) must be square of some natural number. I used fabs(sqrt(..)-(int)sqrt(..))<=EPS and other variants, but.. without success.

In Pascal I used frac(sqrt(..)) and have got AC.
Why? What should I do to solve problem in C?

Calc 8*k-7 in long long...