Finding cliques
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A classical problem in graph theory involves finding cliques. Consider an
undirected graph with n vertices. A clique is a complete subgraph - i.e., 
it is a set of k nodes such that all possible edges between these k nodes exist.
This problem asks you if a clique exists in a given graph. To make things simple
your are told that k=4, i.e., you are looking for a clique of size 4.

Input
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The input will contain multiple instances. The first line of each instance has two 
integers - the number of nodes n and the number of edges m. The second line has m pairs
of numbers each denoting an undirected edge. Assume that the nodes are labeled 1 through n. 
You may assume there are no repeats in the edge list. You are also given that n<20, m < 100.


The end of the input will be indicated by a line containing 0 0. This case 
should not be processed.

Output
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The output should contain one line for each case. Each line should have a
YES or a NO, depending on whether there is a clique of size 4 or not.

Sample input
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4 6
1 2 2 3 3 4 4 1 1 3 2 4
0 0

Sample output
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YES