Pascal :

var
n,i,j:longint;
a:array [0..40] of int64;
begin
 a[0]:=0;a[1]:=3;a[2]:=9;
 for i:=3 to 40 do 
  a[i]:=a[i-1]*2+a[i-2];
      readln(j);
     writeln(a[j]);
 
end.