#include <bits/stdc++.h>
using namespace std;
int main()
{
int m,n,s=0,a=0;
cin>>m>>n;
for(int i=m;i<=n;i++){
	a=i;
	bool t=0;
	if(i==0||i==1)  t=1;
	for(int j=2;j*j<=a;j++){
		if(a%j==0){
			t=1;
			break;
			
		}
	}if(t==0) s++;
	
}
cout<<s<<" ";
return 0;

}

/**************************************************************
	Problem: 1063
	User: mc002
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/