#include<bits/stdc++.h>
using namespace std;
using ll = long long;

vector<bool> IsPrime;

void Sieve(int max){
    if(max + 1 > IsPrime.size()){
        IsPrime.assign(max + 1, true);
    }
    IsPrime[0] = false;
    IsPrime[1] = false;
    for(int i = 2; i * i <= max; i++){
        if(IsPrime[i]){
            for(int j = 2; i * j <= max; j++){
                IsPrime[i * j] = false;
            }
        }
    }
    return;
}

int main(){
    int L, R; cin >> L >> R;
    ll ans = 0;
    Sieve(3 * R);
    for(int a = L; a <= R; a++){
        if(IsPrime[a])++ans;
        if(a < R && IsPrime[2 * a + 1])++ans;
    }
    cout << ans << endl;
}