，怎么在 cout << sum;的后面就结束程序啊，break好像不OK...

#include <iostream>
using namespace std;
int main() {
	int k, sum = 0, N = 0;
	cin >> k;
	for (int i = 1; ; i++) {
		for (int j = 1; j <= i; j++) {
			if (N < k) {
				sum += i;
				N++;
			}
			else {
				cout << sum;
		}
		}
	}
	return 0;
}