How to solve this UTF-8 encoding C problem?

In my class, we were given this problem. I have no clue how to solve it.

"The program below counts the number of characters in a file, assuming the file is encoded as ASCII. Modify the program so that it counts the number of characters in a file encoded as UTF-8"

#include <stdbool.h>
#include <stdio.h>
typedef unsigned char BYTE;
int main(int argc, char *argv[])
{
    if (argc != 2)
    {
        printf("Usage: ./count INPUTn");
        return 1;
    }
    FILE *file = fopen(argv[1], "r");
    if (!file)
    {
        printf("Could not open file.n");
        return 1;
    }
    int count = 0;
    while (true)
    {
        BYTE b;
        fread(&b, 1, 1, file);
        if (feof(file))
        {
            break;
        }
        count++;
    }
    printf("Number of characters: %in", count);
}

Can anyone help me solve this?

First     Last      Number of
Code      Code      bytes in   Byte 1    Byte 2    Byte 3    Byte 4
Point     Point     encoding 
--------  --------  ---------  --------  --------  --------  --------
U+000000  U+00007F          1  0xxxxxxx
U+000080  U+0007FF          2  110xxxxx  10xxxxxx
U+000800  U+00FFFF          3  1110xxxx  10xxxxxx  10xxxxxx
U+010000  U+10FFFF          4  11110xxx  10xxxxxx  10xxxxxx  10xxxxxx