print(int((((8*int(input())+1)**.5)-1)/2))

why the fuck doesnt it work????
i got wa6, who can help me with tests?

Do you know how floating-point numbers work?

Hint: use binary search

Edited by author 24.10.2014 14:27

10^600 gives "OverflowError: int too large to convert to float" :)

You can use Python library "decimal"