#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, t, ans = "";
  cin >> s >> t;
  int k = 1;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == t[i])
      ans += s[i];
    else {
      if (k == 1)
        ans += s[i];
      else
        ans += t[i];
      k *= -1;
    }
  }
  if (k == -1)
    cout << "impossible";
  else
    cout << ans;
}
