Arithmetic operation in pointer using C

Hi friends,
I tried out something like this which is given below in VC++ by referring Dennis M. Ritchie (ANSI C second edition) book examples.

struct Key *low = &tab[0];
struct Key *high = &tab[n];
struct Key *mid;

mid = low + (high-low) / 2; // Error: cannot convert from 'int' to 'struct Key *'


My question is how they are using this expression mid = low + (high-low) / 2;?. I don't know whether I misunderstood the concept or there is some difference between ANSI C and VC++. Please anyone help me in this.

Regards,
S.Shanmuga Raja