import java.util.*;
import java.io.*;
public class Anoisyclass {
    public static void main(String[] args) throws IOException{
        // TODO code application logic here
        BufferedReader br = new BufferedReader (new InputStreamReader(System.in));
        int n = Integer.parseInt(br.readLine());
        int m = Integer.parseInt(br.readLine());
        int c = n;
        ArrayList<Integer>[]adj = new ArrayList[n+1];
        int[] cnt = new int[n+1];
        for (int i = 1; i<=n;i++){
            adj[i] = new ArrayList<Integer>();
        }
        for (int i = 0; i<m; i++){
            StringTokenizer st = new StringTokenizer (br.readLine());
            int a = Integer.parseInt(st.nextToken());
            int b = Integer.parseInt(st.nextToken());
            adj[a].add(b);
            cnt[b]++;
        }
        Queue<Integer> Q = new LinkedList<Integer>();
        for (int i = 1; i<=n; i++){
            if(cnt[i]==0) Q.add(i);
        }
        while (!Q.isEmpty()){
            int a = Q.remove();
            c--;
            for (int i=0;i<adj[a].size();i++){
                int p = adj[a].get(i);
                cnt[p]--;
                if (cnt[p]==0) {
                    Q.add(p);
                }
            }
        }
        if (c==0) System.out.println("Y");
        else System.out.println("N");

    }

}