#include <iostream>
#include <vector>
#include <math.h>
using namespace std;
typedef long long LL;
int IsPrime(LL n)
{
	int i;
	if (n < 2) {
		return 0;
	}
	else if (n == 2) {
		return 1;
	}
	if (n % 2 == 0) {
		return 0;
	}
	double sqrtNum = sqrt((double)n);
	for (i = 3; i <= sqrtNum; i += 2) {
		if (n % i == 0) {
			return 0;
		}
	}
	return 1;
}
int main()
{
	LL L, R,i;
	cin >> L >> R;
	int N = R - L + 1;
	vector<LL> X(N);
	X[0] = L;
	int l = L + 1;
	for (i = 1; i < N; i++) {
		X[i] += X[i-1] + l;
		l++;
	}
	int Count = 0;
	while (X.size() > 0) {
		N = X.size();
		for (i = 0; i < N; i++) {
			if (IsPrime(X[i])) {
				Count++;
			}
			if (i > 0) {
				X[i] -= X[0];
			}
		}
		X.erase(X.begin());
	}
	cout << Count << endl;
	return 0;
}