def f(lst):
    xs = sorted(lst)
    ps = []
    i = 0
    j = len(lst) - 1
    while i < j:
        if xs[i] + xs[j] == 100:
            ps.append((xs[i], xs[j]))
            i += 1
            j -= 1
        elif xs[i] + xs[j] > 100:
            j -= 1
        elif xs[i] + xs[j] < 100:
            i += 1
    return set(ps)