var
  n,i:byte;
  a,b,c:qword;
begin
  while not eof do begin
  readln(n);
  if n=1 then writeln(1)
         else if n=2 then writeln(2)
                     else begin
    a:=1;b:=2;
    for i:=3 to n do begin
      c:=a+b;
      a:=b;
      b:=c;
      end;
      writeln(c);
    end;
  end;
end.

那里错了？