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();
    static int[] NMi => ReadLine().Split().Select(c => int.Parse(c) - 1).ToArray();
    static int[][] NMap(int n) => Enumerable.Repeat(0, n).Select(_ => NMi).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (l, r) = (c[0], c[1]);
        var plist = GetMinPDivList(r * 2);
        var ans = 0;
        for (var i = l; i <= r; ++i)
        {
            if (i > 1 && plist[i] == i)
            {
                ++ans;
            }
            if (i + 1 <= r && plist[i * 2 + 1] == i * 2 + 1)
            {
                ++ans;
            }
        }
        WriteLine(ans);
    }
    static int[] GetMinPDivList(int max)
    {
        var minPDivList = new int[max + 1];
        for (var i = 0; i <= max; ++i) minPDivList[i] = i;

        for (var i = 2; i * i <= max; ++i) if (minPDivList[i] == i)
        {
            for (var j = i * i; j <= max; j += i) if (minPDivList[j] == j)
            {
                minPDivList[j] = i;
            }
        }
        return minPDivList;
    }
}