#include <bits/stdc++.h>
using namespace std;
char s[100001], t[100001], p[100001];
int main() {
  int n, cnt = 0;
  scanf("%s%s", s, t);
  n = strlen(s);
  for (int i = 0; i < n; i++)
    if (s[i] == t[i])
      p[i] = s[i];
    else
      p[i] = cnt ? t[i] : s[i], cnt ^= 1;
  p[n] = '\0';
  if (!cnt)
    printf("%s\n", p);
  else
    puts("impossible");
  return 0;
}
