#include <bits/stdc++.h>
using namespace std;
char s[100007], y[100007];
int main() {
  scanf(" %s %s", s, y);
  int diff = 0;
  for (int i = 0; s[i] != 0; ++i) {
    if (s[i] != y[i]) diff++;
  }
  if (diff % 2 == 1) {
    printf("impossible\n");
  } else {
    for (int i = 0; s[i] != 0; ++i) {
      if (s[i] == y[i]) {
        printf("%c", s[i]);
      } else {
        printf("%c", (diff % 2 == 0 ? s[i] : y[i]));
        diff--;
      }
    }
    printf("\n");
  }
}
