#include <bits/stdc++.h>
using namespace std;
int main() {
  string a, b;
  int x = 0;
  cin >> a >> b;
  for (int i = 0; i < a.length(); i++) {
    if (a[i] != b[i]) x++;
    if (x % 2 == 0) a[i] = b[i];
  }
  if (x % 2 != 0)
    cout << "impossible";
  else
    cout << a;
  return 0;
}
