#include <bits/stdc++.h>
using namespace std;

int l, r, m = 0;
bool f[1501];
vector<long long> p = { 2 };

bool isprime(int t) {
	if (t == 1)
		return 0;

	for (long long k : p) {
		if (t == k)
			return 1;

		if (t % k == 0) {
			return 0;
		}
	}

	return 1;
}

int main()
{
	cin >> l >> r;

	for (long long i = 3; i <= 1500; i += 2) {
		if (!f[i]) {
			p.push_back(i);

			for (long long j = i * i; j <= 1500; j += i * 2) {
				f[j] = 1;
			}
		}
	}

	for (long long i = l; i < r; i++) {
		long long a[2] = { i, i * 2 + 1 };
		
		for(int j = 0; j < 2; j++){
			if (isprime(a[j]))
				m++;
		}
	}

	if (isprime(r))
		m++;

	cout << m << endl;
}