program azshara(input,output);
var
 a:array[1..10000,1..1000,1..2]of longint;
 max:longint;
 i,j,k,w,l:integer;
begin
 assign(input,'azshara.in');assign(output,'azshara.out');
 reset(input);rewrite(output);
 readln(w,l);
 for i:=1 to l do
  for j:=1 to w do
   read(a[i,j,1]);
 for i:=1 to w do a[l,i,2]:=a[l,i,1];

 for i:=l-1 downto 1 do
  for j:=1 to w do begin
   max:=0;
   for k:=-1 to 1 do begin
    if (j+k<1)or(j+k>w)or(a[i+1,j+k,1]=-1) then continue
    else if a[i+1,j+k,2]>max then max:=a[i+1,j+k,2];
   end;
   a[i,j,2]:=max+a[i,j,1];
  end;
 max:=0;
 for i:=1 to w do
  if (a[1,i,1]<>-1)and(a[1,i,2]>max)then max:=a[1,i,2];
 write(max);
 close(input);close(output);
end.