program sas;
var
 a:array[1..2001]of longint;
 i,j,k,m,n,max:longint;
 s:boolean;
 procedure ff(x,y,z:longint);
 var i,j:integer;
 begin
  j:=0;
  writeln(y);
  if x>=n then
    if max<z then max:=z;
  if x<n then
    begin
      if s=true then
        for i:=1 to y do
          begin
            j:=j+a[x+i-1];
            ff(x+i,2*y,z+j);
          end;

      if s=false then
        for i:=1 to y do
            ff(x+i,2*y,z);

    end;
  if s=true then s:=false else s:=true;
 end;
begin
 assign(input,'xoinc.in');
 reset(input);
 assign(output,'xoinc.out');
 rewrite(output);
 readln(n);
 for i:=1 to n do
   read(a[i]);
 k:=a[1];
 s:=false;
 ff(1,1,0);
 writeln(max);
end.