FN.java

 
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
 
/*
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 * and open the template in the editor.
 */
 
/**
 *
 * @author cteam94
 */
public class FN {
    Map<Integer, List<Integer>> map = new HashMap<Integer, List<Integer>>();
 
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
 
 
        while((new FN()).readLine(br));
    }
 
    public boolean readLine(BufferedReader br) throws IOException {
        String line = br.readLine();
        if (line == null) return false;
 
        map = new HashMap<Integer, List<Integer>>();
 
        String[] sp = line.split(" ");
        int n1 = Integer.parseInt(sp[0]);
        int n2 = Integer.parseInt(sp[1]);
 
        for (int i = 1; i <= n1; i++) {
            map.put(i, new ArrayList<Integer>());
        }
 
        for (int i = 0; i < n2; i++) {
            String ln = br.readLine();
            String[] sp2 = ln.split(" ");
            int u = Integer.parseInt(sp2[0]);
            int v = Integer.parseInt(sp2[1]);
 
            List<Integer> l = map.get(u);
            l.add(v);
 
            l = map.get(v);
            l.add(u);
        }
 
//        System.out.println(map);
 
        for (Map.Entry<Integer, List<Integer>> entry : map.entrySet()) {
            int c = 0;
            Integer integer = entry.getKey();
            List<Integer> list = entry.getValue();
            for (Integer v : list) {
                if (isPow(v)) c++;
                if (c >= 4) {
                    System.out.println("YES");
                    return true;
                }
            }
        }
        System.out.println("NO");
 
        return true;
    }
 
 
    public boolean isPow(int x) {
        if (map.get(x).size() == 1) return true;
        return false;
    }
 
 
}