#include <bits/stdc++.h>
    using namespace std;
    #include <math.h>
    #include <iomanip>
    #include <cstdint>
    #include <string>
    #include <sstream>
    template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
    template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
    #define rep(i,n) for (int i = 0; i < (n); ++i)
    typedef long long ll;
    using P=pair<ll,ll>;
    const int INF=1001001001;
    const int mod=998244353;

    void solve(){
        int l,r;
        cin>>l>>r;
        vector<int>seen(2*r+5);
        seen[1]=1;
        for(int i=2;i<=2*r+1;i++){
            if(seen[i]){continue;}
            for(int j=i*2;j<=2*r+1;j+=i){
                seen[j]=1;
            }
        }
        int cnt=0;
        for(int i=l;i<=r;i++){
            if(!seen[i]){cnt++;}
            if(i<r&&!seen[i*2+1]){cnt++;}
        }
        cout<<cnt<<endl;
    }  

    int main(){
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        
        solve();
        
        return 0;
    }