#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, second;
  cin >> s >> second;
  bool a[s.size()], b[s.size()], c[s.size()];
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == '0')
      a[i] = 0;
    else
      a[i] = 1;
  }
  for (int i = 0; i < s.size(); i++) {
    if (second[i] == '0')
      b[i] = 0;
    else
      b[i] = 1;
  }
  bool m = 1;
  for (int i = 0; i < s.size(); i++) {
    if (a[i] == b[i])
      c[i] = a[i];
    else {
      if (m)
        c[i] = a[i];
      else
        c[i] = b[i];
      m = !m;
    }
  }
  if (m) {
    for (int i = 0; i < s.size(); i++) cout << c[i];
  } else
    cout << "impossible";
}
