问可读性返回负值

EN

#include <stdio.h>
#include <cs50.h>
#include <string.h> //getstring
#include <math.h>
#include <ctype.h>


int main (void) 
    {
        string text = get_string ("Text:") ; //get input from user
        int letter = 0, word = 1, sentance = 0; 
        
        for (int i = 0, n = strlen(text); i < n; i++) 
      
        if (isalpha(text[i]))      //identify how many characters are alphabetical
        {
            letter++;
        }
        
        for (int i = 0, n = strlen(text); i < n; i++) 
        if (isspace(text[i]))     //identify how many spces there are 
        {
            word++;
        }
        
        for (int i = 0, n = strlen(text); i < n; i++)
        if ((text[i]) == '!' || (text[i]) == '?' || (text[i]) == '.')
        {
            sentance++;
        }
        
    
         
        float l = (letter / word) *100.00; //average number of letters per 100 words
        float s = (word / sentance) * 100.00; //average number of words per sentance
        float index = 0.0588 * l - 0.296 * s - 15.8; 
        int round_index = round(index);
        
         if (round_index < 16 && round_index > 1)
        {
            printf("Grade %i \n", round_index);
        } 
        else if (round_index >= 16) 
        {
            printf("Grade 16+ \n") ;
        }
        else if (round_index < 1) 
        {
            printf("Before Grade 1 \n") ;
        }
    }