简单 瞬秒

#include <iostream>
using namespace std;
int main()
{
	int k = 1, n, t = 0, sum = 0, i = 0;
	cin >> n;
	while (n != i)
	{
		t++;
		i++;
		sum += k;
		if (t == k)
		{
			k++;
			t = 0;
		}
	}
	cout << sum << endl;
}