#include <bits/stdc++.h>
using namespace std;
int main() {
  string s1, s2, t = "";
  cin >> s1 >> s2;
  int k = 0;
  for (int i = 0; i < s1.length(); i++) {
    if (s1.at(i) == s2.at(i))
      t = t + s1.at(i);
    else if (k % 2 == 0) {
      t = t + s1.at(i);
      k++;
    } else {
      t = t + s2.at(i);
      k++;
    }
  }
  if (k % 2 != 0)
    cout << "impossible";
  else
    cout << t << endl;
  return 0;
}
