function problem_a(id, pr, n, a):
    i := id
    s := 0

    while i < n:
        s += a[i]
        i += pr

    left := 2 * id + 1
    right := left + 1

    if left < pr:
        s += receive(left)
    if right < pr:
        s += receive(right)

    if id == 0:
        print(s)
    else:
        send((id - 1) / 2, s)