#include<bits/stdc++.h>
using namespace std;

bool check (int n)
{
	for (int i = 2; i*i <= n; i++)
	{
		if (n % i == 0)
		 return false;
	}
	return true;
}


int cut;
int t;
int main ()
{
	int a,b;
	cin >> a >> b;
	if (a == 1 || b == 1)
	{
		cut--;
	}else if ( a > b)
	{
		t = a;
		a = b;
		b = t;
	}


	for (int i = a; i <= b; i++)
	{
		if (check(i)){
			cut++;
		}
		
	}
	cout << cut;

}