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

int l, r, m = 0, N;
bool f[2000001];

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

	f[1] = 1;
	N = r * 2 + 1;

	for (long long i = 2; i <= N; i++) {
		if (!f[i]) {

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

	for (long long i = l; i <= r; i++) {
		if(!f[i])
			m++;

		if (i < r && !f[i * 2 + 1])
			m++;
	}

	cout << m << endl;
}