题解 | 被3整除

#include<iostream>
using namespace std;

int main() {
    long long l, r;
    long long count = 0;
    cin >> l >> r;
    for (long long i = l; i <= r; i++) {
        long long m = 0;
        for (long long j = 1; j <= i; j++) {
            m = m + j;
        }
        if (m % 3 == 0) {
            count++;
        }
    }
    cout << count << endl;
    return 0;
}