#include<bits/stdc++.h>
using namespace std;
using Int = long long;
//INSERT ABOVE HERE
signed main(){
  string a,b;
  cin>>a>>b;
  if(a.size()!=b.size()) return cout<<(a.size()>b.size()?a:b)<<endl,0;
  Int n=a.size();
  for(Int i=0;i<n;i++){
    if(a[i]==b[i]) continue;
    if(a[i]=='4'&&b[i]=='7') cout<<a<<endl;
    else if(a[i]=='7'&&b[i]=='4') cout<<b<<endl;
    else if(a[i]<b[i]) cout<<b<<endl;
    else cout<<a<<endl;
    return 0;
  }
  return 0;
}