-module(sum).

-export([sum_m_n/2, sum_m_n_i/2]).

sum_m_n(M, N) when M > N -> 0;
sum_m_n(M, N)            -> M + sum_m_n(M+1, N).

% The same function reimplemented using tail-recursion
sum_m_n_i(M, N, Acc) when M > N -> Acc;
sum_m_n_i(M, N, Acc)            -> sum_m_n_i(M+1, N, Acc + M).

% We define a convenience function
sum_m_n_i(M, N) -> sum_m_n_i(M, N, 0).