#include<iostream> #include<string> #include<string.h> #include<algorithm> #include<stdio.h> #include<cmath> #include<vector> #include<utility> #define FOR(i, a, b) for(int i=(a);i<=(b);i++) #define RFOR(i, a, b) for(int i=(a);i>=(b);i--) #define MOD 1000000007 using namespace std; int main(void) { int n; int total = 0; cin >> n; FOR(i, 1, n) { total += i; } cout << total << endl; return 0; }