fn.cpp

#include <iostream>
#include <cstdio>
#include <cstring>
#include <vector>
#include <stack>
 
using namespace std;
 
bool visited[10000];
 
bool findBunny(vector<vector<int> > & gr, int start){
	stack<int> s, depth;
	int paws = 0, maxDepth = 0;
 
	s.push(start);
	depth.push(0);
 
	while(!s.empty()){
		int n = s.top(), d = depth.top();
		s.pop();
		visited[n] = true;
		bool leaf = true;
 
		if(d > maxDepth) maxDepth = d;
 
		//printf("node %d: ", n);
		for(unsigned i = 0; i < gr[n].size(); i++){
			if(!visited[gr[n][i]]){
				//printf("%d, ", gr[n][i]);
				s.push(gr[n][i]);
				depth.push(d + 1);
				leaf = false;
			}
		}
		//printf("\n");
 
 
		if(leaf){
			paws++;
		}
	}
 
	if(maxDepth >= 2){
		paws++;
	}
 
	//printf("paws: %d\n", paws);
 
	return paws > 3;
}
 
void solve(int n, int m){
	vector<vector<int> > gr(n + 1);
	int a, b;
 
	memset(visited, 0, 10000 * sizeof(bool));
 
	for(int i = 0; i < m; i++){
		scanf("%d %d ", &a, &b);
		gr[a].push_back(b);
		gr[b].push_back(a);
	}
 
	for(int i = 1; i <= n; i++){
		if(!visited[i]){
			if(findBunny(gr, i)){
				printf("YES\n");
				return;
			}
		}
	}
 
	printf("NO\n");
}
 
int main(){
	int n, m;
 
	while(scanf("%d %d ", &n, &m) == 2){
		solve(n, m);
	}
 
	return 0;
}