题解

#include<bits/stdc++.h>
using namespace std;
int a[1010];  

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

	return 0;
}