int truncate(char *s, int n) {
    char new_str[strlen(s)];
    strncpy(new_str, s, n);
    new_str[strlen(s) - 1] = '\0';
    s = new_str;

    return strlen(s) - n;
}

Modify s so it becomes the first n characters of s, and return how many letters in s were removed