#include<bits/stdc++.h>
using namespace std;
bool prime(int x)
{
	for(int j=3;j*j<=x;j+=2){
		if(x%j==0)return 0;
	}
	return 1;
}
int main()
{
	int l,r;
	int ans=0;
	scanf("%d%d",&l,&r);
	if(l==1)l+=2;
	if(l%2==0){
		if(l==2)ans++;
		l++;
	}
	if(r%2==0)r--;
	for(int i=l;i<=r;i+=2)
		if(prime(i))ans++;
	printf("%d",ans);
	return 0;
}