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#include <iostream>
#include <set>
#include <map>
#include <vector>
#include <algorithm>

using namespace std;

#define li long long

char prime[50000501];

int main() {
	int n = 50000500;
	vector<char> prime(n + 1, true);
	prime[0] = prime[1] = false;
	vector<li> p;
	for (li i = 2; i <= n; ++i)
		if (prime[i]) {
			p.push_back(i);
			if (i * 1ll * i <= n)
				for (li j = i * i; j <= n; j += i)
					prime[j] = false;
		}
	li a, b;
	cin >> a >> b;
	cout << (upper_bound(p.begin(), p.end(), b) - p.begin()) - (lower_bound(p.begin(), p.end(), a) - p.begin());

	return 0;
}