An isometry is just a mapping from one metric space to another such that the distance between two points in the domain is equal to the distance between the image of those two points in the range. In other words if is an isometry we have that . In your case we have that and so, assuming this is the standard case, we have that .
So, as Plato mentioned we have that for every so choosing we have that
Many neat things can be seen about isometries. They are clearly Lipschitz, and thus uniformly continuous. They are injective. etc.