given that a+b=c, a+c=d, and b=c+d where d is a positive integer what is the greatest value of a

I know Cphill already answered this, however, I was looking for a numerical value for a.

http://web2.0calc.com/questions/given-that-a-b-c-a-c-d-and-b-c-d-where-d-is-a-positive

Using CPhill's answer:

a = -d

but d is a positive integer, so minimum value of d is 1;

so maximum value of a = -d <-- so a = -1