program azshara;

  var
     n,m,ans:longint;
     f:array[0..1,0..10001] of longint;

procedure init;
  var
     i,j,num,k,d:longint;

 begin
  readln(m,n);
  f[0][0]:=-1; f[0][m+1]:=-1;
  f[1][0]:=-1; f[1][m+1]:=-1;
  for i:=1 to n do
    begin
    k:=i mod 2;
    for j:=1 to m do
      begin
      read(num);
      if num=-1 then f[k][j]:=-1
      else
             for d:=-1 to 1 do
             if f[1-k][j+d]<>-1
                then if f[1-k][j+d]+num>f[k][j]
                      then f[k][j]:=f[1-k][j+d]+num;
      end;
    end;

 end;

procedure print;
 var
    i:longint;

 begin
  if n mod 2=0
  then begin
       for i:=1 to m do
       if f[0][i]>ans then ans:=f[0][i]
       end
 else
       for i:=1 to m do
       if f[1][i]>ans then ans:=f[1][i];
  writeln(ans);
 end;

begin
  assign(input,'azshara.in');
  assign(output,'azshara.out');
  reset(input); rewrite(output);
  init;
  print;
  close(input); close(output);
end.