#include <iostream>
#include <cmath>

// 判断一个数是否为素数的函数
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;

    // 遍历 M 到 N 之间的所有数
    for (int i = M; i <= N; ++i) {
        if (isPrime(i)) {
            ++count;
        }
    }

    std::cout << count << std::endl;

    return 0;
}    
/**************************************************************
	Problem: 1063
	User: fuyijun
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/