#include<stdio.h>
int main()
{
	int l, r;
	scanf("%d %d", &l, &r);
	int prime[2000006];
	int i, j;
	for (i = 0; i < 2000006; i++)
		prime[i] = 1;
	prime[0] = prime[1] = 0;
	int ans = 0;
	for (i = 0; i < 2000006; i++)
	{
		if (prime[i] == 0)
			continue;
		if (l <= i && i <= r)
			ans++;
		if (i % 2 > 0 && l <= i / 2 && (i + 1) / 2 <= r)
			ans++;
		for (j = 2 * i; j < 2000006; j += i)
			prime[j] = 0;
	}
	printf("%d\n", ans);
	return 0;
}