Integer overflow and choosing types

An int holds values up to about 2.1 * 10^9, and a long long up to about 9.2 * 10^18. Sums of many large values, and products of two values around 10^5, already exceed int. Overflow does not give an error: the value silently wraps around and your answer is wrong.

Multiplying two int variables overflows before the result is stored, even if you store it in a long long, so write 1LL * a * b. Doubles (floating point numbers) cannot hold every large integer exactly, so avoid them for exact answers. A common habit is to use long long for every sum and answer.