#include <bits/stdc++.h>
int main() {
  char St1[100001];
  char St2[100001];
  char St3[100001];
  int i;
  scanf(" %s %s", St1, St2);
  int size = strlen(St1);
  int count = 0;
  for (i = 0; i < size; i++) {
    if (St1[i] != St2[i]) {
      count++;
    }
  }
  if (count % 2 != 0) {
    printf("impossible\n");
  } else {
    int counter = 0;
    for (i = 0; i < size; i++) {
      if (St1[i] == St2[i]) {
        St3[i] = '0';
      } else {
        counter++;
        if (count / 2 >= counter) {
          St3[i] = St1[i];
        } else {
          St3[i] = St2[i];
        }
      }
      printf("%c", St3[i]);
    }
  }
  return 0;
}
