#include <bits/stdc++.h>
using namespace std;
string s;
string t;
int main() {
  int i, dem, kt;
  dem = 0;
  cin >> s;
  cin >> t;
  for (i = 0; i < s.length(); i++)
    if (s[i] != t[i]) dem++;
  kt = 0;
  if (dem % 2 == 1)
    cout << "impossible";
  else
    for (i = 0; i < s.length(); i++) {
      if (s[i] != t[i] && kt == 1)
        cout << s[i], kt = (kt + 1) % 2;
      else if (s[i] != t[i] && kt == 0)
        cout << t[i], kt = (kt + 1) % 2;
      else if (s[i] == t[i])
        cout << s[i];
    }
  return 0;
}
