#include <bits/stdc++.h>
using namespace std;
char s[100001], t[100001], p[100001];
int n, kc;
void nhap();
void danhdau();
void xuat();
int main() {
  nhap();
  danhdau();
  xuat();
  cin.get();
  return 0;
}
void nhap() {
  int i;
  const int k = 100000;
  cin >> s;
  cin >> t;
  n = strlen(s);
}
void danhdau() {
  int i, k = -1;
  kc = 0;
  for (i = 0; i < n; i++)
    if (s[i] != t[i]) {
      kc++;
      if (k >= 0)
        if (s[k] == s[i])
          p[i] = (p[k] + 1) % 2 + 48;
        else
          p[i] = p[k];
      else
        p[i] = '0';
      k = i;
    } else {
      p[i] = '0';
    }
}
void xuat() {
  int i;
  if (kc % 2)
    cout << "impossible";
  else
    for (i = 0; i < n; i++) cout << p[i];
}
