#include<iostream>
#include<cmath>
#include<cstring>


using namespace std;

bool a[100005];
int prime[10000], idx = 0, n, m;
int qprime[10000];


int main(void) {

    cin >> m;
    cin >> n;

    a[1] = 1;

    for (int i = 1; i < 100005; i++) {

        if (a[i] == 0) {

            prime[idx] = i;
            idx++;

        }


        for (int j = 0; j < idx && prime[j] * i <= 100000; j++) {

            a[prime[j] * i] = 1;

            if (i % prime[j] == 0) {

                break;

            }

        }


    }

    int count = 0, i = 0;

    while (prime[i] <= m) {

        i++;

    }

    while (prime[i] <= n) {

        count++;
        i++;

    }

    cout << count;

    return 0;

}