简单

#include<bits/stdc++.h>
using namespace std;

int main() {
    int n;
    cin >> n;
    int a[60] = {};
    a[1] = 2;
    for (int i = 2; i <= n; i++)
        a[i] = a[i - 1] + i;
    cout << a[n];
	return 0;
}