#include<iostream>//in out 
using namespace std;
int sum=0,ans=0;
int d=0;
const int maxn=10001;
int q[maxn];
int w[maxn];
bool zhi(int a,int ans){
	int i=a;
	if(i==1||i==0)return false;
	for(int j=2;j*j<=i;j++){
		if(i%j==0)return false;
	}
	return true;
}
bool zhi2(int a){
	int i=a;
	if(i==1||i==0)return false;
	for(int j=2;j<i;j++){
		if(i%j==0)return false;
	}
	return true;
}
int main(){
	int a,b,c=1;
	cin>>a>>b;
	for(int i=a;i<=b;i++){
		if(zhi(i,ans)){
			sum++;
			w[c]=i;
			c++;
		}
	}
	cout<<ans;
	return 0;
}