#include <stdio.h>
#include <iostream>
using namespace std;

int prime[10000001] = {};
int nt[10000001] = {};

void init(long long n_max)
{
    prime[0] = prime [1] = 1;
    for (long long o = 2 ; o < n_max ; o++)
    {
        if(prime[o] == 0)
        {
            nt[o] = nt[o-1] + 1;
            for (long long k = o * o ; k < n_max ; k += o)
            {
                prime[k] = 1;
            }
        }
        else
        {
            nt[o] = nt[o-1];
        }
    }
}

int main()
{
#ifndef ONLINE_JUDGE
   freopen ("input.txt", "r", stdin);
   freopen ("output.txt", "w", stdout);
#endif
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    long long a,b;
    cin >> a >> b;
    init(b + 1);
    cout << nt[b] - nt[a-1];
    return 0;
}