#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;
    for(ll A=L;A<=R-1;A++){
        ll P=2*A+1;
        bool C=true;
        for(ll i=2;i*i<=P;i++){
            if(P%i==0){
                C=false;
                break;
            }
        }
        if(C)an++;
    }
    for(ll A=L;A<=R;A++){
        ll P=A;
        bool C=true;
        for(ll i=2;i*i<=P;i++){
            if(P%i==0){
                C=false;
                break;
            }
        }
        if(C)an++;
    }
    cout<<an<<endl;
}