/********include********/

	#include <cstdio>
	#include <cstring>
	#include <iostream>
	#include <string>
	#include <cmath>
	#include <bitset>
	#include <vector>
	#include <map>
	#include <set>
	#include <queue>
	#include <deque>
	#include <algorithm>
	#include <complex>
	#include <unordered_map>
	#include <unordered_set>
	#include <random>
	#include <cassert>
	#include <fstream>
	#include <utility>
	#include <functional>
	#define popcount __builtin_popcount
	using namespace std;

	/***/
	//#include <iomanip>
	//#include <cmath>
	#include <bits/stdc++.h>

	/********define********/

	#define rep(i,x) for(long long i=0;i<x;i++)
	#define repn(i,x) for(long long i=1;i<=x;i++)
	#define rrep(i,x) for(long long i=x-1;i>=0;i--)
	#define rrepn(i,x) for(long long i=x;i>1;i--)
	#define REP(i,n,x) for(long long i=n;i<x;i++)
	#define REPN(i,n,x) for(long long i=n+1;i<x;i++)

	#define pr printf
	#define re return
	#define mod 1000000007
	//#define mod 998244353
	#define INF 1e18+5//19桁
	const double PI=3.14159265358979323846;
	#define fi first
	#define se second
	#define MAX(a,b) (((a)>(b))?(a):(b))
	#define MIN(a,b) (((a)<(b))?(a):(b))

	typedef long long int ll;
	typedef pair<long long, long long> P;


	/********よく使う関数********/

	// 素因数分解
	vector<pair<long long, long long> > prime_factorize(long long n) {
		vector<pair<long long, long long> > res;
		for (long long p = 2; p * p <= n; ++p) {
			if (n % p != 0) continue;
			int num = 0;
			while (n % p == 0) { ++num; n /= p; }
			res.push_back(make_pair(p, num));
		}
		if (n != 1) res.push_back(make_pair(n, 1));
		return res;
	}

	// 約数列挙
	vector<long long> calc_divisor(long long n) {
		vector<long long> res;
		for (long long i = 1LL; i*i <= n; ++i) {
			if (n % i == 0) {
				res.push_back(i);
				long long j = n / i;
				if (j != i) res.push_back(j);
			}
		}
		sort(res.begin(), res.end());
		return res;
	}

	//最大公約数
	ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }

	//最大公約数 複数個
	ll gcd(const vector<ll> &v) {
		ll ret = v[0];
		for (ll i = 1; i < v.size(); i++)
			ret = gcd(ret, v[i]);
		return ret;
	}

	//最小公倍数
	ll lcm(ll x, ll y) { return x / gcd(x, y) * y; }

	//最小公倍数 複数個
	ll lcm(const vector<ll> &v) {
		ll ret = v[0];
		for (ll i = 1; i < v.size(); i++)
			ret = lcm(ret, v[i]);
		return ret;
	}

	/********変数宣言********/
	//vector<long long> g[200020];
	vector<pair<long long,long long>> g[200020];
	ll s[200020];
	bool used[200020];
	//bool dp[100005];

	ll A,B,C,D,H,K,M,N,Q,R,S,T,W,X,Y;
	double dA,dB,dC,dH,dK,dM,dN,dQ,dR,dS,dT,dW,dX,dY;


	int main()
	{
		cin.tie(0);
		ios::sync_with_stdio(false);
		
		/********よく使う********/
		
		//std::sort(v.begin(),v.end());//昇順ソート
		//std::sort(v.begin(),v.end(),std::greater<long long>());//降順ソート
		//v.erase(unique(v.begin(), v.end()), v.end());
		//vector<ll> v(N, 0);
		
		//vector<pair<long long, long long> > p(N);
		//sort(p.begin(), p.end());
		//sort(p.begin(), p.end(),greater<pair<long long,long long> >());
		
		//priority_queue<long long, vector<long long>, greater<long long> > que;
		//priority_queue<long long> que;
		
		//map<ll, ll, std::greater<ll>> mp;
		
		//deque<long long> dque;
		
		//string s;
		//wk=(ll)stoi(s);

		//set<ll> st;
		//stack<long long> sta;   
		//queue<long long> que;
		//s.insert(0, " ");
		//reverse(s.begin(),s.end());

		//do{
		//}while(next_permutation(v.begin(),v.end()));


		/******初期化***********/

		long long ans,sum,cnt,cnt1,flg,flg1,flg2;
		
		long long max,max1;
		long long min,min1;
		long long wk,wk1;
		max=max1=wk=wk1=sum=ans=cnt=cnt1=flg=flg1=flg2=0;
		min=min1=INF;

		double dwk,dwk1,dsum,dsum1,dans,dans1;
		dwk=dwk1=dsum=dsum1=dans=dans1=0;

		
		cin >> N;
		//rep(i,N){
			//cin >> v[i];
		//}
		pr("%lld",N*(N-1)/2);
		re 0;

		/******出力関連***********/
		//pr("%lld\n",N);
		//printf("%lld",(ll)ceil(dB/dA));
		//puts("Yes");
		//文字列の出力
		//std::cout << s << std::endl;
		//printf("%.12f\n",ret);
		//cout << sum << '\n';
		//pr("%02lld:%02lld",wk/60,wk%60);
		
		/******CF***********/
		/*
		cin >> T;
		while(T--){
			cin >> N >> K;
			std::vector<long long> v(N);	
			// 配列入力1
			/*
			for(long long i=0; i<N; i++){
				std::cin >> v[i];
			}
			*/
			//cout << sum << '\n';
			//文字列の出力
			//std::cout << s << std::endl;
		//}
		//re 0;
		
		/******よく使う***********/
		
		//素因数分解
		/*
		auto pf = prime_factorize(v[i]);
		for(auto p:pf){
			if(p.se!=wk){		
			}
		}
		*/
		//約数列挙
		/*
		vector<long long> div = calc_divisor(M);
		
		// M の約数 d であって、d * N <= M となる最大の d を求める
		long long res = 1;
		for (auto d : div) {
			if (d * N <= M) res = max(res, d);
		}
		*/

		//最小公倍数
		/*
		std::vector<ll> v(N);
		rep(i, N) {
			cin >> v[i];
		}	
		cout << lcm(v) << endl;
		return 0;
		*/

		//最小公約数
		/*
		std::vector<ll> v(N);
		rep(i, N) {
			cin >> v[i];
		}	
		cout << gcd(v) << endl;
		return 0;
		*/

		//素数判定
		/*
		for(int i=x; ; i++){
			bool dame=0;
			for(int j=2; j*j<=i; j++){
				if(i%j==0){
					dame=1;
					break;
				}
			}
			if(!dame){
				cout<<i<<endl;
				return 0;
			}
		}
		*/
		//set典型
		/*
		std::set<long long> st;
		for(long long i=0; i<N; i++){
			std::cin >> v[i];
			if(st.find(v[i])==st.end()){
				st.insert(v[i]);
			}
			else{
				st.erase(v[i]);
			}
		}

		//if(ceil(sqrt(wk)==floor(sqrt(wk))))
		//do{
		//}while(next_permutation(v.begin(),v.end()));

		/******出力関連***********/
		//pr("%lld\n",N);
		//printf("%lld",(ll)ceil(dB/dA));
		//puts("Yes");
		//文字列の出力
		//std::cout << s << std::endl;
		//printf("%.12f\n",ret);
		//cout << sum << '\n';
		//pr("%02lld:%02lld",wk/60,wk%60);
		

		re 0;
		
	}


Measured memory_bytes: 4992