#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, t, res;
  cin >> s >> t;
  int n = s.length();
  int difference = 0;
  for (int i = 0; i < n; i++) {
    if (s[i] != t[i]) {
      difference++;
      if (difference % 2 == 0)
        res += s[i];
      else
        res += t[i];
    } else {
      res += s[i];
    }
  }
  if (difference % 2 == 0)
    cout << res;
  else
    cout << "impossible";
}
