public class Main {


  private static void solve() {
    int n = ni();
    int m = ni();
    int[] from = new int[m];
    int[] to = new int[m];
    for (int i = 0; i < m; i ++) {
      from[i] = ni() - 1;
      to[i] = ni()  -1;
    }
    

    int[][] g = packD(n, from, to);
    int[] a = sortTopologically(g);

    long[] dp = new long[n];
    for (int v : a) {
      for (int next : g[v]) {
        dp[next] = Math.max(dp[next], dp[v] + 1);
      }
    }
    System.out.println(dp[a[n -1]]);
  }

  public static int[][] packD(int n, int[] from, int[] to){ return packD(n, from, to, from.length);}
  public static int[][] packD(int n, int[] from, int[] to, int sup)
  {
      int[][] g = new int[n][];
      int[] p = new int[n];
      for(int i = 0;i < sup;i++)p[from[i]]++;
      for(int i = 0;i < n;i++)g[i] = new int[p[i]];
      for(int i = 0;i < sup;i++){
          g[from[i]][--p[from[i]]] = to[i];
      }
      return g;
  }
  
  public static int[] sortTopologically(int[][] g)
  {
      int n = g.length;
      int[] ec = new int[n];
      for(int i = 0;i < n;i++){
          for(int to : g[i])ec[to]++;
      }
      int[] ret = new int[n];
      int q = 0;
      
      // sources
      for(int i = 0;i < n;i++){
          if(ec[i] == 0)ret[q++] = i;
      }
      
      for(int p = 0;p < q;p++){
          for(int to : g[ret[p]]){
              if(--ec[to] == 0)ret[q++] = to;
          }
      }
      // loop
      for(int i = 0;i < n;i++){
          if(ec[i] > 0)return null;
      }
      return ret;
  }
  
  public static void main(String[] args) {
    new Thread(null, new Runnable() {
      @Override
      public void run() {
        long start = System.currentTimeMillis();
        String debug = args.length > 0 ? args[0] : null;
        if (debug != null) {
          try {
            is = java.nio.file.Files.newInputStream(java.nio.file.Paths.get(debug));
          } catch (Exception e) {
            throw new RuntimeException(e);
          }
        }
        reader = new java.io.BufferedReader(new java.io.InputStreamReader(is), 32768);
        solve();
        out.flush();
        tr((System.currentTimeMillis() - start) + "ms");
      }
    }, "", 64000000).start();
  }

  private static java.io.InputStream is = System.in;
  private static java.io.PrintWriter out = new java.io.PrintWriter(System.out);
  private static java.util.StringTokenizer tokenizer = null;
  private static java.io.BufferedReader reader;

  public static String next() {
    while (tokenizer == null || !tokenizer.hasMoreTokens()) {
      try {
        tokenizer = new java.util.StringTokenizer(reader.readLine());
      } catch (Exception e) {
        throw new RuntimeException(e);
      }
    }
    return tokenizer.nextToken();
  }

  private static double nd() {
    return Double.parseDouble(next());
  }

  private static long nl() {
    return Long.parseLong(next());
  }

  private static int[] na(int n) {
    int[] a = new int[n];
    for (int i = 0; i < n; i++)
      a[i] = ni();
    return a;
  }

  private static char[] ns() {
    return next().toCharArray();
  }

  private static long[] nal(int n) {
    long[] a = new long[n];
    for (int i = 0; i < n; i++)
      a[i] = nl();
    return a;
  }

  private static int[][] ntable(int n, int m) {
    int[][] table = new int[n][m];
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < m; j++) {
        table[i][j] = ni();
      }
    }
    return table;
  }

  private static int[][] nlist(int n, int m) {
    int[][] table = new int[m][n];
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < m; j++) {
        table[j][i] = ni();
      }
    }
    return table;
  }

  private static int ni() {
    return Integer.parseInt(next());
  }

  private static void tr(Object... o) {
    if (is != System.in)
      System.out.println(java.util.Arrays.deepToString(o));
  }
}


Measured memory_bytes: 45872