Menentukan bilangan prima atau tidak

#include<stdio.h>
#include<stdlib.h>
int main()
{
    int a,nilai,b=0;
    scanf("%d",&a);

    if(a == 2 || a == 3 || a == 5 || a == 7)
    {
        b = 1;
    }
    else
    {
        if(a % 2 == 0 || a == 1)
        {
            b = 0;
        }
        else
        {
            if(a % 3 == 0)
            {
                b = 0;
            }
            else if(a % 5 == 0)
            {
                b = 0;
            }
            else if(a % 7 == 0)
            {
                b = 0;
            }
            else
            {
                b = 1;
            }
        }
    }

    if(b == 1)
    {
        printf("ya");
    }
    else
    {
        printf("tidak");
    }
    return(0);
}