#include <bits/stdc++.h>
using namespace std;
string s, t;
string Gao() {
  int n = s.length(), d;
  string answer(n, '0');
  d = 0;
  for (int i = 0; i < n; ++i) {
    if (s[i] != t[i]) {
      if (d > 0) {
        answer[i] = s[i];
        --d;
      } else {
        answer[i] = t[i];
        ++d;
      }
    }
  }
  if (d) {
    return "impossible";
  }
  return answer;
}
int main() {
  while (cin >> s >> t) {
    cout << Gao() << endl;
  }
  return 0;
}
