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

long long l, r, m = 0;
bool f[2000001];
vector<long long> p = { 2 };

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

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

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

	for (long long i = l; i <= r; i++) {
		for (long long j = i; j <= i + 2; j++) {
			long long n = (i + j) * (j - i + 1) / 2;
			bool f = 0;

			for (long long k : p) {
				if (n % k == 0) {
					f = 1;
					break;
				}

				if (k > sqrt(n))
					break;
			}

			if (!f)
				m++;
		}
	}

	cout << m << endl;
}