test.c

#include <stdio.h>
#include <ctype.h>

int main() {
    int num;
    char input;
    int charFrequency[26] = {0};
    
    scanf("%d", &num);

    for (int i = 0; i < num; ++i) {
        scanf(" %c", &input);
        input = tolower(input);
        charFrequency[input - 'a']++;
    }

    for (int i = 0; i < 26; ++i) {
        if (charFrequency[i] > 0) {
            printf("%c: %d\n", 'a' + i, charFrequency[i]);
        }
    }

    return 0;
}