#include <bits/stdc++.h>
using namespace std;
int main() {
  string s1, s2, s;
  cin >> s1 >> s2;
  int i = 0;
  int k = 0;
  while (i < s1.size()) {
    if (s1[i] != s2[i]) {
      k++;
      if (k & 1)
        if (s1[i] == '1')
          s += "0";
        else
          s += "1";
      else if (s2[i] == '1')
        s += "0";
      else
        s += "1";
    } else
      s += s1[i];
    i++;
  }
  if (k % 2 != 0)
    cout << "impossible";
  else
    cout << s;
  return 0;
}
