%Обычная рекурсия

sum(N,0) :- N < 2.
sum(2,2) :- !.

sum(N,S) :- N mod 2 =:= 1, N1 is N-1, sum(N1,S).
sum(N,S) :- N1 is N-2, sum(N1, S1), S is S1+N.


%Хвостовая рекурсия

sum(N,S) :- sum(N,S,0).

sum(N,0, _) :- N < 2.
sum(2,S, PS) :- S is PS + 2, !.

sum(N,S, PS) :- N mod 2 =:= 1, !, N1 is N-1, sum(N1,S, PS).
sum(N,S,PS) :- N1 is N-2, PS1 is PS+N, sum(N1, S, PS1).