#include <iostream>

#include <string>

#include <sstream>

#include <cmath>

using namespace std;

int main()

{

int a, b;

while (cin >> a >> b) {

int sum = 0;

for (int i = a; i <= b; i++) {

int flag = 1;

for (int j = 2; j <= sqrt(i); j += (j == 2 ? 1 : 2))

if (i % j == 0) { flag = 0; break; }

if (i != 1 && flag == 1) sum++;

}

cout << sum << endl;

}

return 0;

}