program ex1;
     var   n:real;
     function f(n:real):real;
  begin
   if n=1 then f:=1
    else if n=2 then f:=1
     else if n=3 then f:=2
       else f:=f(n)+f(n-1)+f(n-2);
  end;
  begin
   write('n=');
   readln(n);
   writeln('f=',f(n):1:0);
   readln
  end.