#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1.0);
const long long MAX = 2e9 + 7;
const long long mod = 1e9 + 7;
const long long maxn = 1e5 + 7;
string a, b;
char s[maxn];
int main() {
  cin.sync_with_stdio(false);
  cin >> a >> b;
  int ans = 0;
  for (int i = 0; i < a.size(); i++) {
    if (a[i] == b[i])
      s[i] = a[i];
    else {
      if (ans % 2 == 1)
        s[i] = b[i];
      else
        s[i] = a[i];
      ans++;
    }
  }
  if (ans % 2)
    cout << "impossible" << endl;
  else
    cout << s << endl;
  return 0;
}
