program azshara;
var
f,x:array[1..10000,0..1000] of longint;
w,l:longint;


procedure init;
var
i,j:longint;
begin
assign(input,'azshara.in');
reset(input);
assign(output,'azshara.out');
rewrite(output);
readln(w,l);
for i:=1 to l do
 begin
 for j:=1 to w do read(x[i,j]);
 readln;
 end;
close(input);
end;

procedure main;
var
i,j,t:longint;
begin
for i:=1 to w do if x[l,i]=-1 then f[l,i]:=-1 else f[l,i]:=x[l,i];
for i:=l-1 downto 1 do
 for j:=1 to w do
 begin
 if x[i,j]=-1 then f[i,j]:=-1
  else
  begin
  t:=x[i,j]+f[i+1,j];
  if t<x[i,j]+f[i+1,j+1] then t:=x[i,j]+f[i+1,j+1];
  if t<x[i,j]+f[i+1,j-1] then t:=x[i,j]+f[i+1,j-1];
  f[i,j]:=t;
  t:=0;
  end;
 end;
end;

procedure print;
begin
writeln(f[1,1]);
close(output);
end;

begin
init;
main;
print;
end.