#include <bits/stdc++.h>
int main() {
  char s[100005], t[100005], p[100005];
  scanf("%s", s);
  getchar();
  scanf("%s", t);
  int i, diff = 0, n = strlen(s);
  for (i = 0; i < n; ++i) {
    if (s[i] != t[i]) diff++;
  }
  if (diff & 1) {
    printf("impossible\n");
  } else {
    diff /= 2;
    for (i = 0; i < n; ++i) {
      if (s[i] != t[i] && diff) {
        p[i] = '1' + '0' - s[i];
        diff--;
      } else
        p[i] = s[i];
    }
  }
  p[i] = '\0';
  printf("%s", p);
  return 0;
}
