var a:array[1..1000000] of longint;
    n,i,k,max:longint;
    f,g:text;
begin
 assign(f,'operatii.in');
 reset(f);
 readln(f,n);
 for i:=1 to n do
  read(f,a[i]);
 close(f);
 k:=0;
 max:=0;
 for i:=1 to n do begin
  if (i=n)and(a[i]>max) then k:=k+a[i]
   else if (a[i]>0)and(a[i-1]=0) then max:=a[i]
    else if a[i]>max then max:=a[i]
     else if (a[i]=0)or(i=n) then begin
      k:=k+max;
      max:=0;
      end;
  end;
 assign(g,'operatii.out');
 rewrite(g);
 writeln(g,k);
 close(g);
end.