Which is greater: 355*356 or 354*357?

Well, I'm looking through a means of solving problems and I just want a little more explanation to this method of solving that question.

Represent 354=a, 357=b, 355=c, and 356=d

We now have that 1) a+b=c+d; 2) |b-a|=|d-c|

We want to prove that ab < dc

This is an elementary question maybe but...how did the problem solving leap from multiplication to addition for the first step?

Can anybody clear that up for me - on why it's a+b=c+d because...to me...it doesn't. And I'm not seeing it. D: