#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

#define DONT_KNOW "I_dont_know"
#define NUMBER_OF_LETTERS ('z' - 'a' + 1)

void printIDontKnow()
{
    printf("%s", DONT_KNOW);
    exit(0);
}

int midterm_quiz_q1(char arr[], int n);

// Do not change read_arr!!!
char *read_arr(int *n)
{
    if (scanf("%d", n) != 1)
        return NULL;
    char *arr = malloc((*n) * sizeof(char));
    if (!arr)
        return NULL;
    for (int i = 0; i < *n; i++)
    {
        if (scanf(" %c", arr + i) != 1)
        {
            free(arr);
            return NULL;
        }
    }
    return arr;
}

//Uncomment the first line in main if you don't know the answer.
//Otherwise, do not change main!!!
int main()
{
    // printIDontKnow();

    int n;
    char *arr = read_arr(&n);
    if (!arr)
        return 1;
    printf("%d", midterm_quiz_q1(arr, n));
    free(arr);
    return 0;
}

/*
* This function gets an array of small english letters and returns the number
* of different letters in the array.
* input:
* 1. char arr[]: the array of letters.
* 2. int n: the length of the array.
* output(int):
*   the number of different letters in the array.
 */
int midterm_quiz_q1(char arr[], int n)
{
    int histogram[NUMBER_OF_LETTERS] = {0};
    for (int i = 0; i < n; i++)
    {
        histogram[arr[i] - 'a']++;
    }

    int numOfLetters = 0;
    for (int i = 0; i < NUMBER_OF_LETTERS; i++)
    {
        if (histogram[i] > 0)
        {
            numOfLetters++;
        }
    }

    return numOfLetters;
}