#include<bits/stdc++.h>

using namespace std;
using ll = long long;

vector<bool> is_prime(2000005,true);

ll prime_num(ll a,ll b){
    ll res = 0;
    is_prime[0] = false;is_prime[1] = false;
    for(int i=2;(ll)i <= 2000005;i++){
        if(is_prime[i]){
            for(int j=2*i;(ll)j <= 2000005;j+=i) is_prime[j] = false;
        }
    }

    for(int i=a;i <= b;i++){
        if(is_prime[i]) res++;
    }

    for(int i=a;i < b;i++){
        if(is_prime[2*i+1]) res++;
    }
    return res;
}

int main(){
    ll l,r;
    cin >> l >> r;

    cout << prime_num(l,r) << endl;
    return 0;
}