#ifdef LOCAL111 #else #define NDEBUG #endif #include const int INF = 1e8; using namespace std; #define endl '\n' #define ALL(a) (a).begin(),(a).end() #define SZ(a) int((a).size()) #define FOR(i,a,b) for(int i=(a);i<(b);++i) #define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--) #define REP(i,n) FOR(i,0,n) #define RREP(i,n) for (int i=(n)-1;i>=0;i--) #define RBP(i,a) for(auto& i : a) #ifdef LOCAL111 #define DEBUG(x) cout<<#x<<": "<<(x)< void dpite(T a, T b){ return; } #else #define DEBUG(x) true template void dpite(T a, T b){ for(T ite = a; ite != b; ite++) cout << (ite == a ? "" : " ") << *ite; cout << endl;} #endif #define F first #define S second #define SNP string::npos #define WRC(hoge) cout << "Case #" << (hoge)+1 << ": " #define rangej(a,b,c) ((a) <= (c) and (c) < (b)) #define rrangej(b,c) rangej(0,b,c) template void pite(T a, T b){ for(T ite = a; ite != b; ite++) cout << (ite == a ? "" : " ") << *ite; cout << endl;} template bool chmax(T& a, T b){if(a < b){a = b; return true;} return false;} template bool chmin(T& a, T b){if(a > b){a = b; return true;} return false;} typedef pair P; typedef long long int LL; typedef unsigned long long ULL; typedef pair LP; void ios_init(){ //cout.setf(ios::fixed); //cout.precision(12); #ifdef LOCAL111 return; #endif ios::sync_with_stdio(false); cin.tie(0); } //library typedef int cost_t; class Edge { public: int to; cost_t cost; Edge(){ } Edge(int x,cost_t y){ to = x; cost = y; } bool operator< (const Edge& x) const { return cost < x.cost; } bool operator> (const Edge& x) const { return cost > x.cost; } }; class Graph { private: //const long long int INF = (long long)1e18; vector > v; int n; public: Graph(int x){ n = x; v = vector >(x); } vector& operator[](int x){ return v[x]; } const vector& operator[](int x) const { return v[x]; } int size() const { return n; } void add_edge(int from, Edge e){ v[from].push_back(e); } void add_edge(int from, int to, cost_t cost){ add_edge(from,Edge(to,cost)); } }; vector dijkstra(int from, const Graph& v) { vector dist(v.size(),INF); priority_queue,greater> que; que.push(Edge(from,0)); while(!que.empty()){ Edge e = que.top(); que.pop(); if(dist[e.to] == INF){ dist[e.to] = e.cost; for(auto to : v[e.to]){ if(dist[to.to] == INF) que.push(Edge(to.to, e.cost+to.cost)); } } } return dist; } vector bellmanford(int from, const Graph& g){ vector res(g.size(),INF); res[from] = 0; bool conf = true; int cnt = 0; while(conf){ conf = false; for(int i = 0; i < g.size(); i++){ if(res[i] != INF) for(const auto& e : g[i]){ if(res[e.to] > res[i]+e.cost){ conf = true; res[e.to] = res[i]+e.cost; } } } if(cnt > g.size()+5) return vector(); cnt++; } return res; } Graph prim(const Graph g){ using T = tuple; priority_queue, greater> qu; int n = g.size(); assert(n != 0); vector added(n,false); qu.emplace(0,0,0); Graph res(n); while(!qu.empty()){ int from, to; cost_t cost; tie(cost, from, to) = qu.top(); qu.pop(); if(!added[to]){ added[to] = true; res.add_edge(from, to, cost); res.add_edge(to, from, cost); for(const auto &e : g[to]){ if(!added[e.to]){ qu.emplace(e.cost, to, e.to); } } } } return res; } //liblary int main() { ios_init(); int n,m; cin >> n >> m; vector

edge(m); Graph g(n); REP(i,m){ int a,b; cin >> a >> b; a--; b--; edge[i] = P(a,b); g.add_edge(a,b,1); g.add_edge(b,a,1); } unordered_set s; RBP(e,g[0]){ s.insert(e.to); } //pite(ALL(s)); string ans = "NO"; REP(i,m){ int a = edge[i].F, b = edge[i].S; if(a == 0 or b == 0) continue; DEBUG(a); DEBUG(b); unordered_set as; RBP(e,g[a]){ if(e.to != b and s.count(e.to)){ as.insert(e.to); } } //pite(ALL(as)); if(SZ(as) == 0) continue; RBP(e,g[b]){ DEBUG(e.to); if(e.to != a and s.count(e.to) and !(as.count(e.to) and SZ(as) == 1)){ DEBUG(a); DEBUG(b); DEBUG(e.to);// pite(ALL(as)); ans = "YES"; break; } } //if(ans == "YES") break; //cout << endl; } cout << ans << endl; return 0; }