#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  string s, t;
  cin >> s >> t;
  int n = s.length();
  bool b = true;
  for (int i = 0; i < n; ++i) {
    if (s[i] ^ t[i]) b ^= true;
    if (!b) s[i] = t[i];
  }
  if (b)
    cout << s;
  else
    cout << "impossible";
  return 0;
}
