Finding a loop invariant

Question

the following pseudo code returns the nth Fibonacci number:

 if n = 0 then
    return 0
 a ← 1
 b ← 1
 for i = 1 to n − 1 do
     c ← b − i
     b ← c + a
     a ← c + 2*i + 1
 return b

I am trying to find any loop invariant for the variables a and b, (as i understand it has to be valid before and after the loop, but i can't seem to be able to come up with anything, except maybe b >= i, but I'm not sure that counts as an invariant). As far as I get it, simply saying b = b - i + a wouldn't really mean anything. Any help on the subject would be greatly appreciated.