백준 1292 - 쉽게 푸는 문제

#include <iostream>

using namespace std;

int main()
{
    int A, B;

    cin >> A >> B;

    int count = 1;
    int stack = 0;
    int acc = 0;

    while (stack+count < A-1)
    {
        stack += count;
        acc += count * count;
        count++;
    }
    acc += count * (A-1 - stack);

    int count2 = 1;
    int stack2 = 0;
    int acc2 = 0;

    while (stack2+count2 < B)
    {
        stack2 += count2;
        acc2 += count2 * count2;
        count2++;
    }
    acc2 += count2 * (B - stack2);

    cout << acc2 - acc;

    return 0;
}

B까지 더한 acc - A-1까지 더한 acc를 하여 해결.