#include <bits/stdc++.h>
using namespace std;
double pi = 3.14159265359;
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  string a, b;
  cin >> a >> b;
  string c(a.size(), '0');
  int count = 0;
  for (unsigned int i = 0; i < a.size(); i++)
    if (a[i] == b[i]) c[i] = a[i], count++;
  if ((a.size() - count) % 2)
    cout << "impossible";
  else {
    bool ok = 0;
    for (unsigned int i = 0; i < a.size(); i++)
      if (a[i] != b[i] && !ok)
        c[i] = a[i], ok = 1;
      else if ((a[i] != b[i] && ok))
        c[i] = b[i], ok = 0;
    cout << c << '\n';
  }
  return 0;
}
