program EmberAsh;
var
a:array[0..10000,0..1000]of longint;
f:array[0..10000,0..1000]of longint;
i,j,k,n,m,w,l,max,jj:longint;
fin,fout:text;

procedure try(x,y,s:longint);
var
i,j,k,x1,y1,s1:longint;
begin
if s>max then max:=s;//处理特殊情况
if (x+1<=l)and(y-1>=0)and(a[x+1,y-1]<>-1)and(s+a[x+1,y-1]>f[x+1,y-1]) then
  begin
  x1:=x+1;y1:=y-1;s1:=s+a[x+1,y-1];
  f[x+1,y-1]:=s1;
  if s1>max then max:=s1;
  try(x1,y1,s1);
  end;//左
if (x+1<=l)and(a[x+1,y]<>-1)and(s+a[x+1,y]>f[x+1,y]) then
  begin
  x1:=x+1;y1:=y;s1:=s+a[x+1,y];
  f[x+1,y]:=s1;
  if s1>max then max:=s1;
  try(x1,y1,s1);
  end;//前
if (x+1<=l)and(y+1<=w)and(a[x+1,y+1]<>-1)and(s+a[x+1,y+1]>f[x+1,y+1]) then
  begin
  x1:=x+1;y1:=y+1;s1:=s+a[x+1,y+1];
  f[x+1,y+1]:=s1;
  if s1>max then max:=s1;
  try(x1,y1,s1);
  end;
end;

BEGIN
assign(fin,'azshara.in');reset(fin);
assign(fout,'azshara.out');rewrite(fout);
readln(fin,w,l);
for i:=1 to l do
  for j:=1 to w do
    read(fin,a[i,j]);
jj:=0;
for i:=1 to w do
  if a[1,i]<>-1 then
    begin
    //max:=0;
    //fillchar(f,sizeof(f),0);
    try(1,i,a[1,i]);//x,y,s
    //if (jj=maxlongint)or(jj<max) then jj:=max;
    end;
//writeln(fout,jj);
for i:=1 to w do
  if f[l,i]>jj then jj:=f[l,i];
writeln(fout,jj);
close(fin);close(fout);
END.