#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstring>
#include<vector>
#include<cmath>
#include<queue>
#include<stack>
#include<map>
#define ll long long
using namespace std;
bool zs(int i)
{
	int s=2;
	if(i<2) return 0;
	while(s<i)
	{
		if(i%s==0) return 0;
		s++;
	}
	return 1;
}
int main()
{
	int x,y,t=0;
	cin>>x>>y;
	if(x>y) swap (x,y);
	for(int i=x;i<=y;i++)
	{
		if(zs(i)) t++;
	}
	cout<<t;
}