#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, t;
  cin >> s >> t;
  string fans = "";
  int cnt = 0;
  for (int i = 0; i < (int)(s.length()); ++i)
    if (s[i] != t[i]) ++cnt;
  if (cnt & 1) {
    cout << "impossible";
    return 0;
  }
  int flag = 0;
  for (int i = 0; i < (int)(s.length()); ++i) {
    if (s[i] == t[i])
      fans += "0";
    else {
      fans += (flag ? s[i] : t[i]);
      flag ^= 1;
    }
  }
  cout << fans;
  return 0;
}
