#include <bits/stdc++.h>
#define PB push_back
#define MP make_pair
#define REP(i,n) for (int i=0;i<(n);i++)
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define ALL(a) (a).begin(),(a).end()
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> P;
const int INF=1e9;
bool rec(string a,string b){
      if(a.size()!=b.size())return a.size()>b.size();
      for(int i=a.size()-1;i>=0;i--){
            if(a[i]!=b[i]){
                  if(a[i]=='4'&&b[i]=='7')return true;
                  else if(b[i]=='7'&&b[i]=='4')return false;
                  return a[i]>b[i];
            }
      }
}
int main(){
      string a,b;
      cin>>a>>b;
      if(rec(a,b))cout<<a<<endl;
      else cout<<b<<endl;
}