#include <iostream>
using namespace std;

bool isPrime(int x){
    if(x < 2) return false;
    for(int i = 2; i * i <= x; i++)
        if(x % i == 0) return false;
    return true;
}

int main(){
    int a,b;

    while(cin >> a >> b){

        int l = min(a,b);
        int r = max(a,b);

        int count = 0;

        for(int i = l; i <= r; i++)
            if(isPrime(i)) count++;

        cout << count << endl;
    }
}