#include <bits/stdc++.h>
using namespace std;
int main() {
  int COUNT = 0;
  string S1, S2, S3 = "";
  cin >> S1 >> S2;
  for (int I = 0; I < S1.length(); I++)
    if (S1[I] == S2[I]) COUNT++;
  int count1 = S1.length() - COUNT;
  if (count1 % 2 == 1) {
    cout << "impossible";
    return 0;
  }
  for (int I = 0; I < S1.length(); I++) {
    if (S1[I] == S2[I])
      S3 += S1[I];
    else {
      if (count1 % 2 == 0)
        S3 += S1[I];
      else
        S3 += S2[I];
      count1++;
    }
  }
  cout << S3;
  return 0;
}
