#pragma GCC optimize("O3")
#include<bits/stdc++.h> 
using namespace std;
using ll=long long;
using P=pair<ll,ll>;
template<class T> using V=vector<T>; 
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
const ll inf=(1e18);
//const ll mod=998244353;
const ll mod=1000000007;
const vector<int> dy={-1,0,1,0},dx={0,-1,0,1};
ll GCD(ll a,ll b) {return b ? GCD(b,a%b):a;}
ll LCM(ll c,ll d){return c/GCD(c,d)*d;}
struct __INIT{__INIT(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(20);}} __init;
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }
template<class T>void debag(const vector<T> &a){cerr<<"debag :";for(auto v:a)cerr<<v<<" ";cerr<<"\n";}
template<class T>void print(const vector<T> &a){for(auto v:a)cout<<v<<" ";cout<<"\n";}
vector<bool> prime;
void make(int num){
    prime.resize(num,true);
    prime[0]=false;
    for(int i=0;i*i<num;i++){
        if(prime[i]){
            for(int j=2;(i+1)*j<=num;j++){
                prime[(i+1)*j-1]=false;
            }
        }
    }
}
int main(){
      make(2000005);
      int l,r;
      cin>>l>>r;
      ll ans=0;
      for(int i=1;i<2000005;i++){
            if(prime[i-1]){
                  if(l<=i&&i<=r)ans++;
                  int a=i/2,b=(i+1)/2;
                  if(a!=b&&l<=a&&b<=r)ans++;
            }
      }
      cout<<ans<<"\n";
}