Pascal :

var a:array['a'..'z'] of char; 
    i,n:longint; 
    st:string[3]; 
    ch:char; 
    f:boolean; 
begin
  for ch:='a' to 'z' do
    a[ch]:=' '; 
  a['a']:='a'; 
  readln(n); 
  for i:=1 to n do
    begin
      readln(st); 
      a[st[1]]:=a[st[3]]; 
    end; 
  f:=false; 
  for ch:='a' to 'z' do
   if a[ch]='a' then
     begin write(ch); f:=true; end; 
  if f then writeln
  else writeln('none'); 
end.