:- use_module(library(lists), [reverse/2, last/2]).

exchange(List,Value) :-
	setof(Value,values(List,Value),AllValues),
	last(AllValues,Value).

values(List,Value) :-
	swap(List,LSwapped),
	value(LSwapped,Value).

swap([],[]).
swap([A|R],[A|RestOut]) :-
	swap(R,RestOut).
swap([A,B|R],Out) :-
	reverse_int(A,ARev),
	reverse_int(B,BRev),
	Out = [BRev,ARev|RestOut],
	swap(R,RestOut).

reverse_int(Int,IntRev) :-
	number_codes(Int,IntL),
	reverse(IntL,IntLRev),
	number_codes(IntRev,IntLRev).

value(L,V) :-
	InitialValue = 0,
	value(L,InitialValue,V).

value([],FinalValue,FinalValue).
value([X],InitialValue,FinalValue) :-
	FinalValue is InitialValue + X.
value([X,Y|R],InitialValue,FinalValue) :-
	InitialValue1 is InitialValue + X - Y,
	value(R,InitialValue1,FinalValue).