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