题解

#include<bits/stdc++.h>
#define ll long long                                                                  //防抄袭标记
#define MAXN 100005
#define maxn 1000005
#define MAXM 10000005
#define maxm 100000005
using namespace std;
int a;
int main(){
	cin>>a;
	cout<<(1+a)*a/2;
	return 0;
}