#include <bits/stdc++.h>
using namespace std;
int main() {
  string a, b;
  cin >> a >> b;
  int n = a.length();
  int counter = 0;
  char ans[n];
  for (int i = 0; i < n; i++) {
    if (a[i] != b[i]) {
      counter++;
    }
  }
  if (counter % 2 == 1) {
    cout << "impossible";
    return 0;
  }
  counter = 0;
  for (int i = 0; i < n; i++) {
    if (a[i] != b[i]) {
      counter++;
      if (counter % 2 == 1) {
        if (a[i] == '0')
          cout << 1;
        else
          cout << 0;
      } else
        cout << a[i];
    } else
      cout << a[i];
  }
  return 0;
}
