#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;

//template
#define rep(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define ALL(v) (v).begin(),(v).end()
using ll=long long int;
const int inf = 0x3fffffff; const ll INF = 0x1fffffffffffffff; const double eps=1e-12;
template<typename T>inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template<typename T>inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}

bool Miller(ll n){
   if(n<2 or (n&1)==0)return (n==2);
   ll d=n-1; while((d&1)==0)d>>=1;
   vector<ll> seeds;
   auto MP=[&](ll x,ll t,ll m)->ll{
      ll res=1;
      while(t){
         if(t&1)res=(__int128_t(res)*x)%m;
         x=(__int128_t(x)*x)%m; t>>=1;
      } return res;
   };
   if(n<(1<<30))seeds={2, 7, 61};
   else seeds={2, 325, 9375, 28178, 450775, 9780504};
   for(auto& x:seeds){
      if(n<=x)break;
      ll t=d,y=MP(x,t,n);
      while(t!=n-1 and y!=1 and y!=n-1){
         y=(__int128_t(y)*y)%n; t<<=1;
      }
      if(y!=n-1 and (t&1)==0)return 0;
   } return 1;
}

int main(){
   int L,R;
   cin>>L>>R;

   int res=0;
   rep(a,L,R+1){
      if(Miller(a))res++;
      if(a!=R and Miller(a*2+1))res++;
   }
   cout<<res<<'\n';
   return 0;
}