#include <bits/stdc++.h>
char s[100010], t[100010], a[100010];
int main() {
  scanf("%s%s", s, t);
  int n = strlen(s), cnt = 0;
  for (int i = 0; i < n; ++i) {
    if (s[i] == t[i])
      a[i] = '0';
    else {
      ++cnt;
      a[i] = cnt & 1 ? s[i] : t[i];
    }
  }
  if (cnt & 1)
    printf("impossible\n");
  else
    printf("%s\n", a);
  return 0;
}
