#include<bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,m,n) for(int i=m; i<int(n); ++i)
#define repll(i,m,n) for(ll i=m; i<ll(n); ++i)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define fi first
#define se second

template<typename T> bool chmin(T& a, T b){ if(a > b){a = b; return true;} return false; }
template<typename T> bool chmax(T& a, T b){ if(a < b){a = b; return true;} return false; }

template<typename T> T gcd(T a, T b){ return a % b ? gcd(b, a % b) : b; }
template<typename T> T lcm(T a, T b){ return a / gcd(a, b) * b; }

struct Eratosthenes{

    vector<bool> is_prime;
    vector<int> prime_list;
    vector<int> spf;

    Eratosthenes(int N) : is_prime(N+1, true), spf(N+1, -1) {
        is_prime[1] = false;
        spf[1] = 1;

        for(int p = 2; p <= N; ++p){
            if(!is_prime[p]) continue;
            prime_list.push_back(p);
            spf[p] = p;

            for(int q = p+p; q <= N; q += p){
                is_prime[q] = false;
                if(spf[q] == -1) spf[q] = p;
            }
        }
    }

    vector<pair<int, int>> prime_factrize(int n){
        vector<pair<int, int>> res;
        while(n > 1){
            int p = spf[n], ex = 0;
            while(n % p == 0){
                n /= p;
                ++ex;
            }
            res.push_back({p, ex});
        }
        return res;
    }

    vector<int> divisors(int n){
        vector<int> res({1});
        auto pf = prime_factrize(n);
        for(auto p : pf){
            int sz = res.size();
            for(int i = 0; i < sz; ++i){
                int v = 1;
                for(int j = 0; j < p.second; ++j){
                    v *= p.first;
                    res.push_back(res[i] * v);
                }
            }
        }
        // sort(res.begin(), res.end())
        return res;
    }
};

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    Eratosthenes E(2000100);
    auto is_p = E.is_prime;

    int L, R;
    cin >> L >> R;

    int ans = 0;
    for(int i = L; i <= R; ++i) if(is_p[i]) ++ans;
    for(int i = L; i < R; ++i) if(is_p[i+i+1]) ++ans;
    
    cout << ans << endl;
    return 0;
}