defining mapPairs function

Hello,

I just came across with this question on the exam and can not think of implementing it.

mapPair :: (a -> a -> a) -> [a] -> [a]

such that mapPairs f [x1, x2, x3, x4...] = [f x1 x2, f x3 x4,...]

and if the list contains an odd number of elements, the last one is kept unchanged, for example

mapPairs f [x1, x2, x3] = [f x1 x2, x3]


Any ideas will be appreciated, thanks