#include <bits/stdc++.h>
char s[2][100100], t[100100];
int n;
int main() {
  scanf("%s%s", s[0], s[1]);
  n = strlen(s[0]);
  int m = 0;
  for (int i = 0; i < n; i++) m += (s[0][i] != s[1][i]);
  if (m & 1) {
    printf("impossible\n");
    return 0;
  }
  int x = 0;
  for (int i = 0; i < n; i++) {
    if (s[0][i] == s[1][i])
      t[i] = s[0][i];
    else {
      t[i] = s[x][i];
      x ^= 1;
    }
  }
  for (int i = 0; i < n; i++) printf("%c", t[i]);
  printf("\n");
  return 0;
}
