include<bits/stdc++.h>

using namespace std; bool isprime(int n){

if(n<=1) return false;
for(int i=2;i*i<=n;i++){
	if(n%i==0) return false;
}
return true;

} int main(){

int n,m,cnt=0;
cin>>n>>m;
for(int i=n;i<=m;i++){
	if(isprime(i)){
		cnt++;
	}
}
cout<<cnt;
return 0;

}

9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999‥‥‥‥‥‥‥‥‥‥█████▓▓▓▓▓▓きききききききき

include

include

using namespace std; bool prime(int n){

if(n<=1) return false;
for(int i=2;i<=sqrt(n);i++){
	if(n%i==0){
		return false;
	}
}
return true;

} int main(){

int n,m,c=0;
	cin>>n>>m;
	for(int i=n;i<=m;i++){
		if(prime(i)){
			c++; 
		}
	}
cout<<c<<endl;
return 0;

}