#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, t;
  cin >> s >> t;
  long long count = 0, i;
  for (i = 0; i < s.size(); i++) {
    if (s[i] != t[i]) count++;
    if (count % 2 == 0) s[i] = t[i];
  }
  (count % 2 == 0) ? cout << s : cout << "impossible";
}
