C :: Guessing Game Program - Binary Search

I'm playing with a guessing game program as a personal exercise, but I'm missing a vital piece - the binary search-style code.

"Have the program initially guess 50, and have it ask the user whether the guess is high, low, or correct. If, say, the guess is low, have the next guess be halfway between 50 and 100, that is, 75. If that guess is high, let the next guess be halfway between 75 and 50, and so on."

(We're assuming that the user won't cheat.) I need the average, essentially. As in, (50 + 75) / 2 = 63.. but when I use this method of "guess = (high+low)/2, it just keeps giving me 50. I can't remember what operators I should use to increment the program's response based on the user's input. It's literally a binary search, that needs to go where those TODOs are. If low was chosen, it would have to start by being at least 51, to 100, so I'd have to set that, then find the average.

Code:
#include <stdio.h> Code: #include <ctype.h>
int main(int argc, const char * argv[]) {
int low;
int high;
int guess;
int response;
int toupper ( int );

[Code] ....