#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <cassert>
using namespace std;


int main() {
    int n, m;
    cin >> n >> m;
    assert(m - n > 0 && m - n < 1E6);
    int lst = -1;
    int ans = 0;
    vector<int> primes;
    for (int i = 2; i*i <= 1E9; i++) {
        bool ok = true;
        for (int j = 2; j*j <= i; j++) {
            if (i % j == 0) {
                ok = false;
                break;
            }
        }
        if (ok && i > 1) {
            primes.push_back(i);
        }
    }

    for (int i = n; i <= m; i++) {
        bool ok = true;
        for (int j = 0; j < primes.size() && primes[j]*primes[j] <= i;
             j++) {
            if (i % primes[j] == 0) {
                ok = false;
                break;
            }
        }

        if (ok && i > 1) {
            ans += (lst == i - 2);
            lst = i;
        }
    }
    cout << ans << endl;
    return 0;
}