#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define lc k<<1
#define rc k<<1|1
#define re register
const int N=5e4+10,mod=998244353;
int read()
{
	int x;
	scanf("%d",&x);
	return x;
}
void prt(int x,int op)
{
	if(op==1) cout<<x<<" ";
	if(op==2) cout<<x<<"\n";
}
long long n;
int main()
{
    while(scanf("%lld",&n)!=EOF)
    {
    	cout<<n*(n-1)/2<<"\n";
	}
 	return 0;
}