uses math;
var x,y,n,m,i,ans:longint;
    dp:array[1..1000000]of longint;
     f:text;
begin
 assign(f,'longpath.in');
 reset(f);
 read(f,n,m);
 for i:=1 to m do
  begin
   read(f,x,y);
   dp[y]:=max(dp[y],dp[x]+1);
   if dp[y]>ans
    then ans:=dp[y];
  end;
 close(f);

 assign(f,'longpath.out');
 rewrite(f);
 writeln(f,ans);
 close(f);
end.