#include<bits/stdc++.h>
using namespace std;

bool primenumberjudge(int n);

int main(){
    int n, m;
    int cnt;
    while(cin >> n >> m){
        cnt = 0;
        if(n > m){
            int tmp = n;
            n = m;
            m = tmp;
        }
        for(int i = n; i <= m; i++)
            if(primenumberjudge(i))
                cnt++;
        cout << cnt << endl;
    }
    return 0;
}

bool primenumberjudge(int n){
    if(n == 1 || n == 0)
        return false;
    for(int i = 2; i < n; i++){
        if(n % i == 0)
            return false;
    }
    return true;
}