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

int issu(int n)
{
	int t = sqrt(n);
	int flag = 1;//judgement
	for (int i=2; i<=t; i++)
		if (n % i == 0)
		{
			flag = 0;
			break;
		}
	return flag;
}
int main()
{
	int a,b;
	while (cin >> a>>b)
	{
		int count=0;
		if(a>b)
			swap(a,b);
		for(int i=a;i<=b;i++)
			if(issu(i))
				count++;
		cout<<count<<endl;
	}
	return 0;
}