using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    public static void Main()
    {
        var c = NList;
        var ans = 0;
        for (var i = c[0]; i <= c[1]; ++i) if ((c[0] + c[1] + i) % 3 == 0) ++ans;
        WriteLine(ans);
    }
}