#include <bits/stdc++.h>
using namespace std;
char mokh(char a) {
  if (a == '0') return '1';
  return '0';
}
int main() {
  string a, b, s;
  int n;
  cin >> a >> b;
  s = a;
  vector<int> dis;
  n = a.size();
  for (int i = 0; i < n; ++i)
    if (a[i] != b[i]) dis.push_back(i);
  if (dis.size() % 2 == 1)
    cout << "impossible";
  else {
    for (int i = 0; i < dis.size() / 2; ++i) s[dis[i]] = mokh(s[dis[i]]);
    cout << s;
  }
}
