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