Cannot apply indexing with [] to an expression of type "int"

Question

int n = int.Parse(textBox2.Text);

int Primes[n + 1];

for (int i = 0; i <= Sqrt(n); i++)

       Primes[i] = 1;

Primes[0] = 0;

Primes[1] = 0;

for(int i = 2; i <= n; i++)

{

     if(Primes[i] == 1)

     {

         for (int j = 2; i * j <= n; j++)

         Primes[i * j] = 0;

     }

}

 The error is the subject!

I marked error code as red! 

Is anyone know why?

Thanks