#include <bits/stdc++.h>
using namespace std;
int num(int n){
    int r,i=1;
    if(n==1){
        r=1;
    }
    else{
        r=num(n-1)+n;
        n--;
    }
    return r;
}
int main(){
    int v;
    cin>>v;
    cout<<num(v);
    return 0;
}
/**************************************************************
	Problem: 1148
	User: luyanchen
	Language: C++
	Result: Wrong Answer
****************************************************************/