include <bits/stdc++.h>
using namespace std;
int main(){
    int n,h=0;
    cin>>n;
    for(int i=n-1;i>=0;i--){
    	h=h+i;
    }
    cout<<h;
}
/**************************************************************
	Problem: 1332
	User: wangyiyang
	Language: C++
	Result: Compile Error
****************************************************************/