#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  string a, b;
  int cou = 0, f = 0;
  cin >> a >> b;
  for (int i = 0; i < a.size(); i++) {
    if (a[i] != b[i]) cou++;
  }
  if (cou % 2 == 1) {
    cout << "impossible\n";
    return 0;
  }
  for (int i = 0; i < a.size(); i++) {
    if (a[i] != b[i]) {
      if (f == 0) {
        f = 1;
        cout << a[i];
      } else {
        f = 0;
        cout << b[i];
      }
    } else {
      cout << a[i];
    }
  }
  return 0;
}
