#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001

int main(){
	
	vector<bool> f(3000001,true);
	f[1] = false;
	for(int i=2;i<f.size();i++){
		if(!f[i])continue;
		for(int j=i*2;j<f.size();j+=i)f[j] = false;
	}
	
	long long a,b;
	cin>>a>>b;
	
	int ans = 0;
	for(int i=a;i<b;i++){
		if(f[i+i+1])ans++;
	}
	for(int i=a;i<=b;i++){
		if(f[i])ans++;
	}
	
	cout<<ans<<endl;
	
	
	return 0;
}