#include <bits/stdc++.h>
using namespace std;
int main(int argc, const char* argv[]) {
  string first;
  string second;
  cin.clear();
  getline(cin, first);
  cin.clear();
  getline(cin, second);
  char answer[100000];
  long long length = first.length();
  int same = 0;
  int notSame = 0;
  for (int i = 0; i < length; i++) {
    if (first[i] == second[i]) {
      same++;
      answer[i] = first[i];
    } else {
      notSame++;
      if (notSame % 2 == 0) {
        answer[i] = first[i];
      } else {
        answer[i] = second[i];
      }
    }
  }
  if (notSame % 2 != 0) {
    printf("impossible");
  } else {
    for (int i = 0; i < length; i++) printf("%c", answer[i]);
  }
  printf("\n");
  return 0;
}
