import java.util.*;
import java.lang.*;

class Main
{
    public static int largestTriangleNumberSmallerThan(int x) {
        int n = (int) (Math.sqrt(8*x + 1) - 1)/2;
        return n*(n+1)/2;
    }
    
	public static void main (String[] args) throws java.lang.Exception
	{
		System.out.println(largestTriangleNumberSmallerThan(65));
	}
}