#include <iostream>
using namespace std;
int main() {
    int a = 0, b = 0, cnt = 0;
    cin >> a >> b;
    for (int n = a; n <= b; n++){
        bool a = true;
        for (int i = 2; i * i <= n; i++) {
            if (n % i == 0) {
                a = false;
                break;
            }
        }
        if(a)
            cnt++;
    }
    cout << cnt << endl;
    return 0;
}