让天去选算法吧！

#include <algorithm>
#include <iostream>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
using namespace std;

int main() {
    int n;
    cin >> n;
    if (rand() % 2) {
        cout << n * (n + 1) / 2; // clever than Gaussian
    } else {
        int sum = 0; // not clever than Gaussian
        for (int i = 1; i <= n; i ++) {
            sum += i;
        }
        cout << sum;
    }
    return 0;
}