#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#define _GLIBCXX_DEBUG
using namespace std;
using std::cout;
using std::cin;
using std::endl;
using ll=long long;
using ld=long double;
ll ILL=1167167167167167167;
const int INF=2100000000;
ll mod=1e9+7;
#define rep(i,a) for (ll i=0;i<a;i++)
template<class T> using _pq = priority_queue<T, vector<T>, greater<T>>;
template<class T> ll LB(vector<T> &v,T a){return lower_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> ll UB(vector<T> &v,T a){return upper_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> bool chmin(T &a,const T &b){if(a>b){a=b;return 1;}else return 0;}
template<class T> bool chmax(T &a,const T &b){if(a<b){a=b;return 1;}else return 0;}
template<class T> void So(vector<T> &v) {sort(v.begin(),v.end());}
template<class T> void Sore(vector<T> &v) {sort(v.begin(),v.end(),[](T x,T y){return x>y;});}
void yneos(bool a){if(a) cout<<"Yes\n"; else cout<<"No\n";}


//a以下の素数を列挙、計算量:Nlog(log(N))
vector<long long> Eratosthenes(long long a){
    if(a<2) return {};
    vector<long long> p(a+1),ans;
    p[0]=1,p[1]=1;
    long long k=2;
    while(k*k<=a){
        if(p[k]==0){
            ans.push_back(k);
            for(long long i=2;i*k<=a;i++){
                p[i*k]=1;
            }
        }
        k++;
    }
    while(k<=a){
        if(p[k]==0) ans.push_back(k);
        k++;
    }
    return ans;
}

//  rainy ~ 雨に打たれて ~
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	
	auto p=Eratosthenes(2e6+1000);
    ll L,R;
    cin>>L>>R;
    if(L==R){
        int b=LB(p,L);
        if(p[b]==L) cout<<1<<endl;
        else cout<<0<<endl;
        return 0;
    }
    cout<<UB(p,R)-LB(p,L)+UB(p,R*2-1)-LB(p,L*2+1)<<endl;
}