Erlang

-module(l).
-export([prog/1]).
 
find([], _, _) -> -1;
find([H|T], N, Sum) ->
    if
        H == Sum ->
            N;
        true ->
            find(T, N + 1, Sum)
    end.
 
prog(List) ->
    Sum = lists:nth(1, List) + lists:last(List),
    io:format("~p~n", [find(List, 1, Sum)]).