#include <bits/stdc++.h>
using namespace std;
const int maxn = 1000 + 10;
int main() {
  int f = 0;
  string s, ss, ans;
  cin >> s >> ss;
  ans = "";
  int len = s.length();
  for (int i = 0; i < len; i++) {
    if (s[i] == ss[i]) {
      ans += '1';
    } else {
      ans += (f == 1 ? s[i] : ss[i]);
      f = f == 0 ? 1 : 0;
    }
  }
  if (f == 0) {
    cout << ans << endl;
  } else {
    cout << "impossible" << endl;
  }
  return 0;
}
