#include <bits/stdc++.h>
using namespace std;
int main () {
    int L, R;
    cin >> L >> R;
    vector<bool> sosuu((R + 1) * 2, true);
    sosuu[0] = sosuu[1] = false;
    for (int i = 2; i < (R + 1) * 2; i ++) {
        if (sosuu[i]) {
            for (int j = 2; j * i < (R + 1) * 2; j ++) {
                sosuu[i * j] = false;
            }
        }
    }
    int ans = 0;
    for (int i = L; i <= R; i ++) {
        ans += (int)sosuu[i];
        if (i > L) {
            ans += (int)sosuu[i * 2 - 1];
        }
    }
    cout << ans << endl;
}