（最长路径）给定一个有向无环图，每条边长度为 1，求图中的最

var
  n, m, i, j, a, b, head, tail, ans : longint;
  graph : array[0..100, 0..100] of longint;
// 用邻接矩阵存储图 
  len, queue, degree : array[0..100] of longint;
    // len[]记录以各结点为终点的最长路径长度 
    // queue[]存放拓扑排序结果 
    // degree[]记录每个结点的入度 
begin
  read(n, m);
  for i := 0 to n - 1 do
    for j := 0 to n - 1 do
      graph[i][j] := 0;
  for i := 0 to n - 1 do
    degree[i] := 0;
  for i := 0 to m - 1 do
  begin
    read(a, b);
    graph[a][b] := 1;
    1;
  end;
  tail := 0;
  for i := 0 to n - 1 do
    if (2) then
    begin
      queue[tail] := i;
      tail := tail + 1;
    end;
  head := 0;
  while (tail < n - 1) do
  begin
    for i := 0 to n - 1 do
      if (graph[queue[head]][i] = 1) then
      begin
        3;
        if (degree[i] = 0) then
        begin
          queue[tail] := i;
          tail := tail + 1;
        end;
      end;
    4;
  end;
  ans := 0;
  for i := 0 to n - 1 do
  begin
    a := queue[i];
    len[a] := 1;
    for j := 0 to n - 1 do
      if (graph[j][a] = 1) and (len[j] + 1 > len[a]) then
        len[a] := len[j] + 1;
    if (5) then
      ans := len[a];
  end;
  writeln(ans);
end.