#include <stdio.h>
#include  <math.h>
int prime(long long x);
int main()
{
	long long x, y,i,count=0;
	scanf("%lld %lld", &x, &y);
	for (i = x; i <= y; i++)
	{
		if (prime(i))
		{
			count++;
		}
	}
	printf("%lld", count);
	return 0;
}
int prime(long long x)
{
	if (x == 1) return 0;
	if (x == 2) return 1;
	long long j = 2;
	while (j * j <= x && x % j != 0) j++;
	if (x % j == 0) return 0;
	else return 1;
}