#include <iostream>
#include <cmath>
bool isPrime(int num) {
    if (num<2) {
        return false;
    }
    for (int i=2;i<=std::sqrt(num);++i) {
        if (num%i==0) {
            return false;
        }
    }
    return true;
}
	int main() {
    int M, N;
    std::cin >> M >> N;
    int count = 0;
    for (int i = M; i <= N; ++i) {
        if (isPrime(i)) {
            ++count;
        }
    }
    std::cout << count << std::endl;
    return 0;
}    
/**************************************************************
	Problem: 1063
	User: panjinyan
	Language: C++
	Result: Accepted
	Time:13 ms
	Memory:2072 kb
****************************************************************/