Finding double primes between two values

I've been stuck on this problem for ages now and don't seem to know how to complete this program. I am a beginner in programming so I only know C. I will admit this is for an assignment and I am not looking for the answer but I would really appreciate a little help with the problem. This is what I have:

#include <stdio.h>
#include <math.h>

void main()
{
    int low, high, n, count,i;

    scanf ("%d%d",&low, &high);

    count=0;

    n=0;

    if (low<=3)
        count=count+1;


    while (n<low)
        {

            n=n+6;

    }

    while (n<high)
    {
     i=1;

       while (i<sqrt(n+1))
        {
        i=i+2;

        if ((n-1)%i==0 || (n+1)%i==0)

        continue;

            else
                count=count+1;
        }

        n=n+6;

    }

 printf("the number of twin primes between %d and %d are %d",low, high, count);

}

Am I using while loops wrong and/or the if statements? I haven't been taught to use for loops so I can't use those. I also have to use the fact that every twin prime besides {3,5} follow the formula 6+/-1.

Thank you for helping me