program ex;
type
 ss=array[0..10000,0..1001]of longint;
var
 a,f:ss;
 f1,f2:text;
 m,n:integer;
procedure init;
 var
  i,j:integer;
 begin
  assign(f1,'azshara.in');
  assign(f2,'azshara.out');
  reset(f1);
  rewrite(f2);
  readln(f1,m,n);
  for i:=1 to n do
   begin
    for j:=1 to m do
      read(f1,a[i,j]);
    readln(f1);
   end;
  for i:=1 to m do
   f[n,i]:=a[n,i];
 end;
procedure main;
 var
  i,j,x,y,z,maxt:longint;
 begin
  maxt:=0;
  for i:=n-1 downto 1 do
   for j:=1 to m do
    if a[i,j]<>-1 then
     begin
      x:=a[i,j]+f[i+1,j+1];
      y:=a[i,j]+f[i+1,j];
      z:=a[i,j]+f[i+1,j-1];
      if x>y then
         if x>z then f[i,j]:=x
                else f[i,j]:=z
         else   if y>z then f[i,j]:=y
                       else f[i,j]:=z;
     end;
  for j:=1 to m do
   if f[1,j]>maxt then maxt:=f[1,j];
  writeln(f2,maxt);
 end;
begin
 init;
 main;
 close(f2);
end.