#include <stdio.h>
#include <cmath>

int main()
{
	int a, b; //b > a
	int x, y, z;
	
	
	scanf("%d", &a);
	scanf("%d", &b);
	
	int sum = b - a;
	
	while ( a < b)
	{
		for (x = 2; x < a; x++)
		{
			if (a % x ==0)
			{
				sum--;
				break;
			}
		}
		a++;
	}
	
	printf("%d", sum);
	
	return 0;
}