#include<bits/stdc++.h>
using namespace std;
int qq(int q){
	int o=1;
	if(q>1){
		for(int i=2;i<q;i++){
			if(q%i==0){
				o=0;
				break;
			}
		}
	}
	else{
		o=0;
	}
	return o;
}
int main()
{
	int i,n,m,s=0;
	cin>>m>>n;
	for(i=m;i<=n;i++){
		if(qq(i)==1)
			s++;
	}
	cout<<s;
return 0;
}
/**************************************************************
	Problem: 1063
	User: houshanglin
	Language: C++
	Result: Accepted
	Time:125 ms
	Memory:2072 kb
****************************************************************/