#include <bits/stdc++.h>
using namespace std;
int main() {
  char s[100005], t[100005];
  int i, count = 0;
  scanf("%s", s);
  scanf("%s", t);
  for (i = 0; s[i]; i++) {
    if (s[i] != t[i]) count++;
  }
  if (count % 2 == 1)
    printf("impossible");
  else {
    for (i = 0; s[i]; i++) {
      if (s[i] == t[i])
        printf("%c", s[i]);
      else {
        if (count % 2 == 0)
          printf("%c", s[i]);
        else
          printf("%c", t[i]);
        count--;
      }
    }
  }
}
