#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,A;
	cin >> L >> R;
	int Count = 0;
	for (A = L; A <= R; A++) {
		if (IsPrime(A)) {
			Count++;
		}
	}
	for (A = L; A < R; A++) {
		if (IsPrime(A+A+1)) {
			Count++;
		}
	}
	cout << Count << endl;
	return 0;
}