#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool read(T &x) {
  int c = getchar();
  int sgn = 1;
  while (~c && c < '0' || c > '9') {
    if (c == '-') sgn = -1;
    c = getchar();
  }
  for (x = 0; ~c && '0' <= c && c <= '9'; c = getchar()) x = x * 10 + c - '0';
  x *= sgn;
  return ~c;
}
int X[] = {-1, -1, -1, 0, 1, 1, 1, 0};
int Y[] = {-1, 0, +1, 1, 1, 0, -1, -1};
int main() {
  string s, t;
  cin >> s >> t;
  string ans;
  int dif = 0;
  int rev = 0;
  ans = s;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == t[i])
      ans[i] = s[i];
    else {
      dif++;
      if (rev == 0) {
        ans[i] = s[i];
        rev = 1;
      } else {
        ans[i] = t[i];
        rev = 0;
      }
    }
  }
  if (dif % 2)
    cout << "impossible" << endl;
  else
    cout << ans << endl;
  return 0;
}
