lst = [0, 1, 3, 8, 9, 10, 12, 13, 14, 17, 18, 20]
w = []
for i in range(0, len(lst) - 1):
    if((lst[i + 1] - lst[i]) > 1):
        w.append(lst[i])
        w.append(lst[i+1])
print(set(w))