#include <iostream>
#include <algorithm>
using namespace std;
using i64 = long long;

class range {private: struct I{int x;int operator*(){return x;}bool operator!=(I& lhs){return x<lhs.x;}void operator++(){++x;}};I i,n;
public:range(int n):i({0}),n({n}){}range(int i,int n):i({i}),n({n}){}I& begin(){return i;}I& end(){return n;}};

void bye(string s) {
  puts(s.c_str());
  exit(0);
}

int main(void) {
  string a, b; cin >> a >> b;
  int n = a.size(),
      m = b.size(),
      ha = max(n, m);
  for(int i=0; i<ha-n; i++) { a = '0' + a; }
  for(int i=0; i<ha-m; i++) { b = '0' + b; }
  for(int i=0; i<ha; i++) {
    if(a[i] == b[i]) { continue; }
    if(a[i] == '4' && b[i] == '7') {
      bye(a);
    }
    if(a[i] == '7' && b[i] == '4') {
      bye(b);
    }
    bye(a[i] > b[i] ? a : b);
  }
  return 0;
}