You are given a [directed graph](https://en.wikipedia.org/wiki/Directed_graph) consisting of n vertices and m edges (each edge is directed, so it can be traversed in only one direction). You are allowed to remove at most one edge from it.

Can you make this graph [acyclic](https://en.wikipedia.org/wiki/Directed_acyclic_graph) by removing at most one edge from it? A directed graph is called acyclic iff it doesn't contain any cycle (a non-empty path that starts and ends in the same vertex).

Input

The first line contains two integers n and m (2 ≤ n ≤ 500, 1 ≤ m ≤ min(n(n - 1), 100000)) — the number of vertices and the number of edges, respectively.

Then m lines follow. Each line contains two integers u and v denoting a directed edge going from vertex u to vertex v (1 ≤ u, v ≤ n, u ≠ v). Each ordered pair (u, v) is listed at most once (there is at most one directed edge from u to v).

Output

If it is possible to make this graph acyclic by removing at most one edge, print YES. Otherwise, print NO.

Examples

Input

3 4
1 2
2 3
3 2
3 1


Output

YES


Input

5 6
1 2
2 3
3 2
3 1
2 1
4 5


Output

NO

Note

In the first example you can remove edge <image>, and the graph becomes acyclic.

In the second example you have to remove at least two edges (for example, <image> and <image>) in order to make the graph acyclic.
import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.BitSet;
import java.util.Calendar;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.PriorityQueue;
import java.util.SortedSet;
import java.util.Stack;
import java.util.StringTokenizer;
import java.util.TreeMap;
import java.util.TreeSet;

/**
 * #
 * @author pttrung
 */
public class D_Edu_Round_36 {

    public static long MOD = 1000000007;

    static boolean[] back, then;
    static int min;

    public static void main(String[] args) throws FileNotFoundException {
        // PrintWriter out = new PrintWriter(new FileOutputStream(new File(
        // "output.txt")));
        PrintWriter out = new PrintWriter(System.out);
        Scanner in = new Scanner();
        int n = in.nextInt();
        int m = in.nextInt();
        ArrayList<Integer>[] map = new ArrayList[n];
        for (int i = 0; i < n; i++) {
            map[i] = new ArrayList();
        }
        int[][] data = new int[m][2];
        for (int i = 0; i < m; i++) {
            data[i][0] = in.nextInt() - 1;
            data[i][1] = in.nextInt() - 1;
            map[data[i][0]].add(i);

        }
        back = new boolean[n];
        then = new boolean[n];

        int re = 0;
        int[] check = new int[n];
        boolean[]stack = new boolean[n];
        for(int i = 0; i < n; i++){
            if(check[i] == 0){

                check[i] = 1;
               re += cal(i, check, stack,data,map);
            }
        }

       // System.out.println(Arrays.toString(back) + " " + Arrays.toString(then));
        if (re <= 1) {
            out.println("YES");
        } else  {


            int z = n + 2;
            int[][] flow = new int[z][z];

            for (int i = 0; i < n; i++) {

                    if(back[i]){
                        flow[i][z - 1] = 100;
                    }
                    if(then[i]){
                        flow[0][i] = 100;
                    }
                    for (int j : map[i]) {
                        int nxt = data[j][1];
                        flow[i][nxt]++;

                    }

            }
            int count = 0;
            int[] pa = new int[z];

            while (count < 2) {

                Arrays.fill(pa, -1);
                pa[0] = 0;
                LinkedList<Integer> q = new LinkedList();
                q.add(0);
                while (!q.isEmpty()) {
                    int node = q.poll();

                    for (int j = 0; j < z; j++) {
                        if (flow[node][j] > 0 && pa[j] == -1) {
                            q.add(j);
                            pa[j] = node;
                        }
                    }
                }
                if (pa[z - 1] != -1) {
                    min = Integer.MAX_VALUE;
                    update(z - 1, pa, flow);
                    count += min;
                    if (min == 0) {
                        break;
                    }
                } else {
                    break;
                }
            }
            //System.out.println(count + " " + Arrays.toString(back));
            if (count == 1) {
                out.println("YES");
            } else {
                out.println("NO");
            }

        }

        out.close();
    }

    static int cal(int index, int []check, boolean[]inStack,int[][]data, ArrayList<Integer>[]map){
        inStack[index] = true;
        int re = 0;
        for(int i : map[index]){
            int nxt = data[i][1];
            if(check[nxt] == 0){
                check[nxt] = 1 + check[index];
                re += cal(nxt, check,inStack, data, map);
            }else if(inStack[nxt]){
                if(check[nxt] < check[index]){
                    //System.out.println(check[nxt] + " " + check[index] + " " + nxt + " " + index);
                    back[index] = true;
                    then[nxt] = true;
                    re++;
                }
            }
        }
        inStack[index] = false;
        return re;
    }


    static void update(int u, int[] pa, int[][] flow) {
        if (pa[u] != u) {
            min = Integer.min(min, flow[pa[u]][u]);
            update(pa[u], pa, flow);
            flow[pa[u]][u] -= min;
            flow[u][pa[u]] += min;
        }
    }


    public static int[] KMP(String val) {
        int i = 0;
        int j = -1;
        int[] result = new int[val.length() + 1];
        result[0] = -1;
        while (i < val.length()) {
            while (j >= 0 && val.charAt(j) != val.charAt(i)) {
                j = result[j];
            }
            j++;
            i++;
            result[i] = j;
        }
        return result;

    }

    public static boolean nextPer(int[] data) {
        int i = data.length - 1;
        while (i > 0 && data[i] < data[i - 1]) {
            i--;
        }
        if (i == 0) {
            return false;
        }
        int j = data.length - 1;
        while (data[j] < data[i - 1]) {
            j--;
        }
        int temp = data[i - 1];
        data[i - 1] = data[j];
        data[j] = temp;
        Arrays.sort(data, i, data.length);
        return true;
    }

    public static int digit(long n) {
        int result = 0;
        while (n > 0) {
            n /= 10;
            result++;
        }
        return result;
    }

    public static double dist(long a, long b, long x, long y) {
        double val = (b - a) * (b - a) + (x - y) * (x - y);
        val = Math.sqrt(val);
        double other = x * x + a * a;
        other = Math.sqrt(other);
        return val + other;

    }

    public static class Point implements Comparable<Point> {

        int x, y;

        public Point(int start, int end) {
            this.x = start;
            this.y = end;
        }

        @Override
        public int hashCode() {
            int hash = 5;
            hash = 47 * hash + this.x;
            hash = 47 * hash + this.y;
            return hash;
        }

        @Override
        public boolean equals(Object obj) {
            if (obj == null) {
                return false;
            }
            if (getClass() != obj.getClass()) {
                return false;
            }
            final Point other = (Point) obj;
            if (this.x != other.x) {
                return false;
            }
            if (this.y != other.y) {
                return false;
            }
            return true;
        }

        @Override
        public int compareTo(Point o) {
            return Integer.compare(x, o.x);
        }
    }

    public static class FT {

        long[] data;

        FT(int n) {
            data = new long[n];
        }

        public void update(int index, long value) {
            while (index < data.length) {
                data[index] += value;
                index += (index & (-index));
            }
        }

        public long get(int index) {
            long result = 0;
            while (index > 0) {
                result += data[index];
                index -= (index & (-index));
            }
            return result;

        }
    }

    public static long gcd(long a, long b) {
        if (b == 0) {
            return a;
        }
        return gcd(b, a % b);
    }

    public static long pow(long a, long b, long MOD) {
        if (b == 0) {
            return 1;
        }
        if (b == 1) {
            return a;
        }
        long val = pow(a, b / 2, MOD);
        if (b % 2 == 0) {
            return val * val % MOD;
        } else {
            return val * (val * a % MOD) % MOD;

        }
    }

    static class Scanner {

        BufferedReader br;
        StringTokenizer st;

        public Scanner() throws FileNotFoundException {
            // System.setOut(new PrintStream(new BufferedOutputStream(System.out), true));
            br = new BufferedReader(new InputStreamReader(System.in));
            //  br = new BufferedReader(new InputStreamReader(new FileInputStream(new File("input.txt"))));
        }

        public String next() {

            while (st == null || !st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (Exception e) {
                    throw new RuntimeException();
                }
            }
            return st.nextToken();
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        public String nextLine() {
            st = null;
            try {
                return br.readLine();
            } catch (Exception e) {
                throw new RuntimeException();
            }
        }

        public boolean endLine() {
            try {
                String next = br.readLine();
                while (next != null && next.trim().isEmpty()) {
                    next = br.readLine();
                }
                if (next == null) {
                    return true;
                }
                st = new StringTokenizer(next);
                return st.hasMoreTokens();
            } catch (Exception e) {
                throw new RuntimeException();
            }
        }
    }
}