#include <stdio.h>

#define MAXLEN 110

int isPrime(int n); //判断一个整数是否是质数，是则返回1，否则返回0
int getResult(char word[]);

int main() {
  char word[MAXLEN];

  scanf("%s", word);
  printf("%d\n", getResult(word));

  return 0;
}

/* 请在这里填写答案 */
int getResult(char word[]) {
  int hashtable[26] = {};

  // Load word into hash table
  for (int i = 0; word[i] != '\0'; i++) {
    hashtable[word[i] - 'a']++;
  }

  // Analyze words
  int max = 0;
  int min = MAXLEN;
  for (int i = 0; i < 26; i++) {
		// If hashtable[i] == 0, the letter is not in the word.
    if (hashtable[i] == 0) {
      continue;
    }
    if (hashtable[i] > max) {
      max = hashtable[i];
    }
    if (hashtable[i] < min) {
      min = hashtable[i];
    }
  }

  if (isPrime(max - min)) {
    return max - min;
  } else {
    return -1;
  }
}