#include <bits/stdc++.h>
using namespace std;
int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int L, R;
    cin >> L >> R;
    vector<int> fac(2000020);
    iota(fac.begin(), fac.end(), 0);
    for(int i = 2; i < 2000020; i++){
        if(fac[i] != i) continue;
        int t = i;
        while(t < 2000020){
            fac[t] = i;
            t += i;
        }
    }
    int ans = 0;
    for(int i = L; i <= R; i++){
        if(i != 1 && fac[i] == i) ans++;
        if(i < R){
            if(fac[2 * i + 1] == 2 * i + 1) ans++;
        }
    }
    cout << ans << endl;
}