#include<bits/stdc++.h>
using namespace std;
#define int long long
#define REP(i,m,n) for(int i=(m);i<(n);i++)
#define rep(i,n) REP(i,0,n)
#define pb push_back
#define all(a) a.begin(),a.end()
#define rall(c) (c).rbegin(),(c).rend()
#define mp make_pair
#define endl '\n'
//#define vec vector<ll>
//#define mat vector<vector<ll> >
#define fi first
#define se second
#define double long double
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll,ll> pll;
//typedef long double ld;
typedef complex<double> Complex;
const ll INF=1e9+7;
const ll MOD=998244353;
const ll inf=INF*INF;
const ll mod=MOD;
const ll MAX=400010;
const double PI=acos(-1.0);
typedef vector<vector<ll> > mat;
typedef vector<ll> vec;


signed main(){
    ll l,r;cin>>l>>r;
    vector<ll>d(2*r);
    REP(i,2,2*r){
        if(d[i]!=0)continue;
        ll k=1;
        while(k*i<2*r){
            if(d[k*i]==0)d[k*i]=i;
            k++;
        }
    }
    ll ans=0;
    if(d[r]==r)ans++;
    REP(i,l,r){
        if(d[i]==i)ans++;
        if(d[2*i+1]==2*i+1)ans++;
    }
    cout<<ans<<endl;
}