#include<iostream>
using namespace std;
int main()
{
	int X, Y, i = 0, k = 2, f=0;
	cin >> X >> Y;
	if (X > Y)
	{
		X = i;
		X = Y;
		Y = i;
	}
	f = X;
	for (; f < +Y; f++)
	{
		for (; k < f; k++)
		{
			if (f % k == 0)
			{
				break;
			}
		}
		if (k == f)
		{
			i++;
		}
		k = 2;
	}
	cout << i << endl;
	return 0;
}