40分，求条，必关

#include <bits/stdc++.h>
using namespace std;
long long l, r;
long long cnt;
long long b[100000010];

int  main() {
	cin >> l >> r;
	for (long long i = 2; i <= r; i++) {
		if (b[i] == 0) {
			for (long long j = 2; j * i <= r; j++) {
				b[j * i] = 1;
			}
		}
	}
	for (long long i = l; i <= r; i++) {
		if (b[i] == 0)
			cnt++;
	}
	cout << cnt;
	return 0;
}