#include <bits/stdc++.h>
using namespace std;
int main() {
  char str1[100001], str2[100001], str3[100001];
  cin >> str1;
  cin >> str2;
  int i = 0, ct = 0;
  while (str1[i] != '\0') {
    if (str1[i] != str2[i]) {
      if (ct % 2 == 0)
        str3[i] = str1[i];
      else
        str3[i] = str2[i];
      ct++;
    } else
      str3[i] = str2[i];
    i++;
  }
  if (ct % 2 != 0)
    cout << "impossible";
  else {
    for (int j = 0; j != i; j++) cout << str3[j];
  }
  return 0;
}
