#include <iostream>
#include <string>
using namespace std;

int main() {
string s1;
string s2;

cin >> s1 >> s2;

if (s1.length() > s2.length()) {
 cout << s1 << endl;
} else if (s1.length() < s2.length()) {
 cout << s2 << endl;
}

for (int i = 0; i < s1.length(); ++i) {
 if (s1[i] == s2[i]) {
 continue;
 }
 if (s1[i] > s2[i]) {
  if (s1[i] == '7' && s2[i] == '4') {
   cout << s2 << endl;
  } else {
   cout << s1 << endl;
  }
 } else {
  if (s1[i] == '4' && s2[i] == '7') {
   cout << s1 << endl;
  } else {
   cout << s2 << endl;
  }
 }
}

}