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

class Main
{
    public static int largestTriangleNumberSmallerThan(int x) {
        int i = 0;
        int last = 0;
        while (true) {
            int triangle = i*(i+1)/2;
            if (triangle > x) return last;
            last = triangle;
            i++;
        }
    }
    
	public static void main (String[] args) throws java.lang.Exception
	{
		System.out.println(largestTriangleNumberSmallerThan(65));
	}
}