using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Nhap N:");

        int N = int.Parse(Console.ReadLine());

        int m = 0, S = 0;

        for (int i = 1; i < N; i++)

        {

            S = S + i;

            if (S >= N) 

            {

                m = i - 1;

                break;

            }

        }       

        Console.WriteLine("m = {0}",m);

    }

}