#include <iostream>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <iomanip>
#include <queue>
#include <algorithm>
#include <numeric>
#include <deque>
#include <complex>
#include <cassert>

using namespace std;

vector<bool> prime;

void erat(long long N){
    prime.resize(N+1, 1);
    prime[0] = 0;
    prime[1] = 0;
    for (long long i=2; i*i<=N; i++){
        if (prime[i]){
            for (long long j=i*2; j <= N; j+=i){
                prime[j] = 0;
            }
        }
    }
}

int main(){

    int L, R, ans=0;
    cin >> L >> R;
    erat(3e6);

    for (int i=L; i<=R; i++){
        if (prime[i]) ans++;
        if (prime[2*i+1] && i+1<=R) ans++;
    }

    cout << ans << endl;

    return 0;
}