数据：
4 4
1 2 2 1 3 4 4 3

ans：1 2 3 4

........................................................................

type
  arr=record
    point,next:longint;
  end;
var
  edge:array[0..500000] of arr;
  head,stack,ans,low,dfn:array[0..5100] of longint;
  m,n,i,j,k,l,s,x,y,t:longint;
function min(a,b:longint):longint;
  begin
  if a>b then exit(b)
     else exit(a);
  end;
procedure tarjan(dep:longint);
  var
    p,po,x:longint;
    f:boolean;
  begin
    inc(t);
    dfn[dep]:=t;
    low[dep]:=t;
    inc(stack[0]);
    stack[stack[0]]:=dep;
    p:=head[dep];
    while p<>0 do
      begin
      po:=edge[p].point;
      if low[po]=-1 then
         begin p:=edge[p].next; continue; end;
      if low[po]=0 then tarjan(po);
      low[dep]:=min(low[dep],low[po]);
      p:=edge[p].next;
      end;
    if dfn[dep]=low[dep] then
       begin
       for p:=stack[0] downto 1 do
         begin
         low[stack[p]]:=-2;
         if stack[p]=dep then break;
         end;
       f:=true;
       for po:=stack[0] downto p do
         begin
         x:=head[stack[po]];
         while x<>0 do
           begin
           if low[edge[x].point]<>-2 then
              begin f:=false; break; end;
           x:=edge[x].next;
           end;
         if not f then break;
         end;
       if f then
          for po:=stack[0] downto p do
            begin
            inc(ans[0]);
            ans[ans[0]]:=stack[po];
            end;
       for po:=stack[0] downto p do
         low[stack[po]]:=-1;
       stack[0]:=p-1;
       end;
  end;
procedure sort(l,r: longint);
var  i,j,x,y: longint;
begin
  i:=l;j:=r;x:=ans[(l+r) div 2];
  repeat
    while ans[i]<x do inc(i);
    while x<ans[j] do dec(j);
    if not(i>j)
       then begin
              y:=ans[i];ans[i]:=ans[j];ans[j]:=y;
              inc(i);dec(j);
            end;
  until i>j;
  if l<j then sort(l,j);
  if i<r then sort(i,r);
end;
begin
  read(n);
  while n<>0 do
    begin
    readln(m);
    for i:=1 to n do
      begin
      head[i]:=0;
      low[i]:=0;
      end;
    s:=0;
    for i:=1 to m do
      begin
      read(x,y);
      inc(s);
      edge[s].point:=y;
      edge[s].next:=head[x];
      head[x]:=s;
      end;
    t:=0;
    ans[0]:=0;
    stack[0]:=0;
    for i:=1 to n do
      if low[i]=0 then tarjan(i);
    if ans[0]=0 then writeln
       else
       begin
       sort(1,ans[0]);
       write(ans[1]);
       for i:=2 to ans[0] do
         write(' ',ans[i]);
       writeln;
       end;
    read(n);
    end;
end.