#include <bits/stdc++.h>
using namespace std;
int main() {
  ios ::sync_with_stdio(false);
  string s, t;
  int x = 0;
  cin >> s;
  cin >> t;
  for (int i = 0; i < s.length(); i++) {
    if (s[i] != t[i]) x++;
    if (x % 2 == 0) {
      s[i] = t[i];
    }
  }
  if (x % 2 == 1)
    cout << "impossible";
  else
    cout << s;
  return 0;
}
