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