For best readability code as follows:

int c = (a - b);

Clearly this even is more apparent when 'a' and 'b' become more elaborate/complex expressions ... as in real code.

To continu the simple examples, the following is also OK, although less OK for more complex expressions:

int c = a - b;

Finally prevent unclear code like (especially for complexer expressions):

int c = a-b;

See also 11@407.