#include <bits/stdc++.h>
using namespace std;
bool odd(int N) {
  while (N < 0) {
    N += 2;
  }
  return N % 2 == 1 ? true : false;
}
bool even(int N) {
  while (N < 0) {
    N += 2;
  }
  return N % 2 == 0 ? true : false;
}
int main() {
  string s, t;
  cin >> s >> t;
  int N = s.size();
  string result = "";
  int num = 0;
  for (int i = 0; i != N; ++i) {
    if (s[i] != t[i]) {
      ++num;
      if (num % 2) {
        result += s[i];
      } else {
        result += t[i];
      }
    } else {
      result += s[i];
    }
  }
  if (even(num)) {
    cout << result;
  } else {
    cout << "impossible";
  }
  return 0;
}
