#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, t;
  cin >> s >> t;
  long long d = 0;
  long long n = s.length();
  for (long long i = 0; i < n; i++) {
    if (s[i] != t[i]) d++;
  }
  if (d % 2 != 0) {
    cout << "impossible";
    return 0;
  }
  long long arr[n];
  for (long long i = 0; i < n; i++) {
    if (s[i] == t[i]) {
      if (s[i] == '1')
        arr[i] = 1;
      else
        arr[i] = 0;
      continue;
    }
    arr[i] = -1;
  }
  long long t1 = 0;
  long long count = d / 2;
  for (long long i = 0; i < n; i++) {
    if (s[i] != t[i]) {
      if (t1 % 2 == 0) {
        t1++;
        arr[i] = s[i] - 48;
      } else {
        t1++;
        arr[i] = t[i] - 48;
      }
      continue;
    }
  }
  for (long long i = 0; i < n; i++) cout << arr[i];
}
