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#include <iostream>
using namespace std;

bool prime(int x) {
	for (int i = 2; i <= sqrt(x); i++) {
		if (x % i == 0) return false;
	}
	return true;
}

int primesumm(int a, int b) {
	int summ = 0;
	for (int i = a; i <= b; i++) {
		if (prime(i) == true) summ += 1;
	}
	return summ;
}



int main() {
	int a;
	int b;

	cin >> a;
	cin >> b;

	cout << primesumm(a, b);



}