KöMaL 2009 C1000

There are 30 people sitting at a round table. Some of them are liars, the others tell the truth. We know that out of the two neighbours of every liar, exactly one is a liar. Each of the 30 people is asked how many liars are sitting next to them. 12 say exactly one and the others say that both of their neighbours are liars. How many liars are there around the table? (Based on a problem from Subcarpathia)