#include <bits/stdc++.h>
using namespace std;
int main() {
  string str[2];
  cin >> str[0] >> str[1];
  int ct = 0;
  bool turn = 0;
  string res;
  for (int i = 0; i < str[0].length(); i++) {
    if (str[0][i] != str[1][i]) {
      ct++;
      if (!turn) {
        res += str[0][i];
        turn = 1;
      } else {
        res += str[1][i];
        turn = 0;
      }
    } else
      res += str[0][i];
  }
  if (ct % 2 == 0)
    cout << res << "\n";
  else
    cout << "impossible\n";
  return 0;
}
