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