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