C program to count number of vowels in a string

#include<stdio.h>

int main()
{
    char a[100];
    int vowel=0, i;

    printf("Write a sentence: \n");
    gets(a);

    for(i=0; a[i]!='\0'; i++)
    {
        if
           ( a[i]== 'a' || a[i]== 'e' ||a[i]== 'i' ||a[i]== 'o' ||a[i]== 'u' || a[i]== 'A' ||a[i]== 'E' ||a[i]== 'I' ||a[i]== 'O' ||a[i]== 'U' )
           {
               vowel++;
           }

           }
       printf("The number of  vowels are : %d" , vowel);


    return 0;
}