#include <bits/stdc++.h>
int arr[1 << 8];
int ans[1 << 8];
using namespace std;
int main() {
  string s1, s2, s;
  cin >> s1 >> s2;
  int d = 0;
  for (int i = 0; i < s1.size(); i++) {
    if (s1[i] != s2[i]) d++;
  }
  if (d % 2 != 0) {
    cout << "impossible";
    return 0;
  }
  bool ch = 1;
  for (int i = 0; i < s1.size(); i++) {
    if (s1[i] == s2[i])
      s += s1[i];
    else if (ch)
      s += s1[i], ch = 0;
    else
      s += s2[i], ch = 1;
  }
  cout << s;
}
