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;

}

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