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