#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <iostream>
#include <queue>
#include <list>
#include <stack>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;

int main(){
  string A, B;
  cin >> A >> B;
  if(A.length() > B.length()){
    cout << A << endl;
  }
  else if(A.length() < B.length()){
    cout << B << endl;
  }
  else {
    bool flg = false;
    rep(i,A.length()){
      if(A[i] == B[i]) continue;
      if(A[i] == '4' && B[i] == '7'){ flg = false; break; }
      else if(A[i] == '7' && B[i] == '4'){ flg = true; break; }
      else{
        if(A[i] < B[i]){ flg = true; break; }
        else { flg = false; break; }
      }
    }
    if(flg) cout << B << endl;
    else cout << A << endl;
  }
  return 0;
}