#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define all(A) A.begin(),A.end()
using vll = vector<ll>;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
using Graph = vector<vector<ll>>;

int main() {
    ll L,R;
    cin>>L>>R;
    ll an=0;
    vector<bool> P(2*1000000+5,true);
    P[0]=P[1]=false;
    rep(i,2e6+5){
        if(P[i]){
            for(ll j=i+i;j<=2e6+5;j+=i){
                P[j]=false;
            }
        }
    }
    for(ll A=L;A<=R-1;A++){
        if(P[2*A+1])an++;
    }
    for(ll A=L;A<=R;A++){
       
        if(P[A])an++;
    }
    cout<<an<<endl;
}