#include<iostream>
#include<cstdio>
#include<cmath>
#include<algorithm> 
#include<string>
#include<cstring>
using namespace std;
int main()
{
    int a, b;
    while (cin >> a >> b) {
        if (a > b)swap(a, b);
        int cnt1,cnt2 = 0;
        for (int i = a; i <= b; i++) {
            cnt1 = 0;
            for (int j = 2; j * j <= i; j++) {
                if (i % j == 0) {
                    cnt1++;
                    break;
                }
            }
            if (cnt1 == 0)cnt2++;
        }
        cout << cnt2 << endl;
    }
    return 0;
}