#include <bits/stdc++.h>
using namespace std;
void init() {}
string a, b, c;
int turn, d;
int main() {
  init();
  cin >> a >> b;
  for (int i = (0); i < (a.size()); ++i) {
    if (a[i] == b[i]) {
      c += a[i];
    } else {
      d++;
      if (turn) {
        c += a[i];
      } else {
        c += b[i];
      }
      turn ^= 1;
    }
  }
  if (d & 1) {
    cout << "impossible"
         << "\n";
  } else {
    cout << c << "\n";
  }
  return 0;
}
