#include <bits/stdc++.h>
int main() {
  char ch1[100001], ch2[100001];
  unsigned long i;
  char c = 1;
  gets(ch1);
  gets(ch2);
  for (i = 0; ch1[i] > 0; i++) {
    if (ch1[i] != ch2[i]) {
      if (!c) ch1[i] = ch2[i];
      c = !c;
    }
  }
  if (c)
    printf("%s\n", ch1);
  else
    printf("impossible\n");
  return 0;
}
