import java.util.Scanner;


public class Main_yukicoder276 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        long n = sc.nextLong();

        if (n == 1) {
        	System.out.println("1");
        } else {
        	System.out.println(n * (n - 1) / 2);
        }

        sc.close();
    }
}