#include <iostream>
#include <algorithm>
#include <functional>
#include <string>
#include <climits>
#include <vector>
#include <numeric>
#include <complex>
#include <map>
#include <bitset>
#include <stack>
#include <queue>
using namespace std;

#define __int64 long long
#define long __int64
#define REP(i,a,b) for(int i=a;i<b;i++)
#define rep(i,n) REP(i,0,n)
const int Vecy[4] = {0,-1,0,1};
const int Vecx[4] = {1,0,-1,0};
const int modd = 1000000007;

int main(){
	long n;
	long ans;
	cin >> n;
	ans = n * (n - 1) / 2;
	
	if(ans == 0){
		cout << 1 << endl;
	}else{
		cout << ans << endl;
	}
	
    return 0;
}