#include <bits/stdc++.h>
using namespace std;
int main() {
  string a, b, c;
  cin >> a >> b;
  int k = 0;
  int z = 1;
  for (int i = 0; i < a.size(); i++) {
    if (a[i] != b[i]) k++;
  }
  if (k % 2) {
    cout << "impossible";
    return 0;
  }
  for (int i = 0; i < a.size(); i++) {
    if (a[i] == b[i])
      c.push_back(a[i]);
    else if (z) {
      c.push_back(a[i]);
      z = 0;
    } else if (!z) {
      c.push_back(b[i]);
      z = 1;
    }
  }
  cout << c << endl;
  return 0;
}
