Java:
int left = ...;
int right = ...;
int middle = (right - left) / 2 + left;
Alternatively:
int middle = (left + right) / 2;
Proof:
(right - left) / 2 + left = (left + right) / 2
right - left + left * 2 = left + right
right + left = left + right
The second method is much simpler to understand and compute.