#include <bits/stdc++.h>
using namespace std;
int main() {
  {
    string a, b, c;
    cin >> a >> b;
    int i, j, t1 = 0;
    for (i = 0; i < a.size(); i++) {
      if (a[i] != b[i]) {
        if (t1 % 2 == 1) a[i] = b[i];
        ++t1;
      }
    }
    if (t1 % 2 == 1)
      cout << "impossible";
    else
      cout << a;
  }
}
