using System;
using System.Linq;

public class Program
{
    static void Main()
    {
        var input = Console.ReadLine();
        var n = Convert.ToInt32(input);
        var total = 0;
        foreach(var i in Enumerable.Range(1, n))
        {
            total += i;
        }
        Console.WriteLine(total.ToString());
    }
}