#include #include #include #include #include #include #include #include #include #include #include #include #define REP(i,n) for(ll i=0;i<(n);i++) #define MOD 1000000007 #define int long long #ifdef int const long long INF = LLONG_MAX / 10; #else const int INF = 1010101010; #endif using namespace std; typedef long long ll; typedef vector> mat; typedef pair P; //typedef pair P; #ifdef LOCAL #define dump(...) \ do { \ std::ostringstream os; \ os << __LINE__ << ":\t" << #__VA_ARGS__ << " = "; \ print_to(os, ", ", "\n", __VA_ARGS__); \ std::cerr << os.str(); \ } while (0) #define dump_(a) \ do { \ std::ostringstream os; \ os << __LINE__ << ":\t" << #a << " = ["; \ print_to_(os, ", ", "]\n", all(a)); \ std::cerr << os.str(); \ } while (0) #else #define dump(...) #define dump_(...) #endif template void print_to(std::ostream &os, const char *, const char *tail, const T &fst) { os << fst << tail; } template void print_to(std::ostream &os, const char *del, const char *tail, const Fst &fst, const Rst &... rst) { os << fst << del; print_to(os, del, tail, rst...); } template void print_to_(std::ostream &os, const char *del, const char *tail, Iter bgn, Iter end) { for (Iter it = bgn; it != end;) { os << *it; if (++it != end) { os << del; } else { os << tail; } } } template void println(const Fst &fst, const Rst &... rst) { print_to(std::cout, "\n", "\n", fst, rst...); } template void print(const Fst &fst, const Rst &... rst) { print_to(std::cout, " ", "\n", fst, rst...); } template void println_(Iter bgn, Iter end) { print_to_(std::cout, "\n", "\n", bgn, end); } template void print_(Iter bgn, Iter end) { print_to_(std::cout, " ", "\n", bgn, end); } // const int MOD = 1000000007; namespace trush { int _ = (std::cout.precision(10), std::cout.setf(std::ios::fixed), std::cin.tie(0), std::ios::sync_with_stdio(0), 0); } int N, M; int Sa, Sb; int used[252525 * 3]; struct edge { int id; int to; }; vector G[510]; bool dfs(int v, int n) { if (n == M) return true; //dump(v, n); for (edge e : G[v]) { if (!used[252525 + e.id] && !used[252525 - e.id]) { used[252525 + e.id] = true; if (dfs(e.to, n + 1)) return true; used[252525 + e.id] = false; } } return false; } signed main() { cin >> N >> M; REP(i,M) { cin >> Sa >> Sb; G[Sa].push_back(edge{(i + 1), Sb}); G[Sb].push_back(edge{-(i + 1), Sa}); } /* REP(i,N) { if (dfs(i, 0)) { cout << "YES" << endl; return 0; } } */ if (dfs(Sa, 0)) cout << "YES" << endl; else cout << "NO" << endl; }