def inc_val(center, i, neg_toggle):
    mult = 1
    if neg_toggle:
        neg_toggle = False
        mult = -1
    else:
        neg_toggle = True
        mult = 1
        i += 1
    val = center + (i * mult)
    return val, i, neg_toggle

i = 0
center = 24
val = center
neg_toggle = False
my_set = {30, 50, 90}
while val not in my_set:
    print(val)
    val, i, neg_toggle = inc_val(center, i, neg_toggle)

24
25
23
26
22
27
21
28
20
29
19