#include <stdio.h>

int main() {
  int arr[26]={}, i;
  char s[15];
  scanf("%s", s);

  for (i=0; s[i]!='\0'; i++) {
    arr[s[i]-'a']++;
  }

  int one, onecnt=0, twocnt=0;
  for (i=0; i<26; i++) {
    if (arr[i]==1) {
      onecnt++;
      one=i;
    } else if (arr[i]>2) {
      twocnt=1;
    }
  }

  if (onecnt>1 || twocnt) {
    printf("Impossible\n");
  } else {
    printf("%c\n", one+'a');
  }
  return 0;
}