#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, j, l, c = 0;
  string str1, str2;
  cin >> str1 >> str2;
  l = str1.length();
  for (i = 0; i < l; i++) {
    if (str1[i] != str2[i]) {
      c++;
      if (c % 2 == 0) {
        str1[i] = str2[i];
      }
    }
  }
  if (c % 2 != 0) {
    printf("impossible\n");
  } else {
    cout << str1;
  }
  return 0;
}
