typedef long long ll;
typedef long double ld;
#include <bits/stdc++.h>
using namespace std;
#define int long long

vector<bool> isprime;
vector<int> Era(int n) {
    isprime.resize(n, true);
    vector<int> res;
    isprime[0] = false; isprime[1] = false;
    for (int i = 2; i < n; ++i) isprime[i] = true;
    for (int i = 2; i < n; ++i) {
        if (isprime[i]) {
            res.push_back(i);
            for (int j = i*2; j < n; j += i) isprime[j] = false;
        }
    }
    return res;
}

signed main(){
    vector<int> primes = Era(3000000);
    ll l,r;
    std::cin >> l>>r;
    ll ans =0;
    for (int i = l-1; i <= r-1; i++) {
        if(isprime[i+1])ans++;
        if(i+2<=r&&isprime[2*i+3])ans++;
    }
    std::cout << ans << std::endl;
}