#include <bits/stdc++.h>
#define rep(i, l, r) for (int i = (l); i < (r); i++)
using namespace std;

typedef long long ll;

int main() {
    vector<bool> prime(2000000, true);
    prime[0] = false;
    prime[1] = false;
    for (int i = 2; i * i < 2000000; i++) {
        if (!prime[i]) continue;
        for (int j = i * 2; j < 2000000; j += i) prime[j] = false;
    }
    int L, R, ans = 0;
    cin >> L >> R;
    rep(i, L, R + 1) if (prime[i]) ans++;
    rep(i, L, R) if (prime[i + i + 1]) ans++;
    cout << ans << endl;
}