#include <iostream>
using namespace std;

const int N = 10000001;
bool prime[N];

int main(){
	
	prime[1] = true; 
	
    for(int i = 2; i < N; i++)
		prime[i] = true;

    for(int i = 2; i * i < N; i++){
        if(prime[i]){
            for(int j = i * i; j < N; j += i)
                prime[j] = false;
        }
    }

    int a,b;

    while(cin >> a >> b){

        int count = 0;

        for(int i = a; i <= b; i++)
            if(!prime[i]) 
				count++;

        cout << count << endl;
    }

    return 0;
}