#include <bits/stdc++.h>
using namespace std;
int main() {
  string x, y;
  cin >> x >> y;
  int c = 0, f = 0;
  for (int i = 0; i < x.length(); i++) {
    if (x[i] != y[i]) {
      c += 1;
      x[i] = (f) ? x[i] : y[i];
      f = !f;
    }
  }
  if (c % 2 == 0) {
    cout << x << endl;
  } else
    cout << "impossible" << endl;
  return 0;
}
