#pragma GCC optimize("Ofast")
#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <queue>
#include <cstdio>
#include <ctime>
#include <assert.h>
#include <chrono>
#include <random>
#include <numeric>
#include <set>
#include <deque>
#include <stack>
#include <sstream>
#include <utility>
#include <array>
using namespace std;
typedef long long int ll;
typedef unsigned long long ull;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll myRand(ll B) {
    return (ull)rng() % B;
}

const int N = 2e6 + 1;
bool is_not_prime[N];

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    is_not_prime[0] = is_not_prime[1] = true;
    int L,R; cin >> L >> R;
    int res = 0;
    for(int i=2;i<N;i++){
        if(!is_not_prime[i]){
            if(L<=i and i<=R)res++;
            for(int j=i+i;j<N;j+=i)is_not_prime[j]=true;
        }
    }
    for(int i=L;i+1<=R;i++){
        if(!is_not_prime[i+i+1])res++;
    }
    cout << res << endl;
}