program azshara;
type
shuzu=array[0..10000,0..10000] of longint;
var
s,f:shuzu;
i,j,k,m,n,x,y,max:longint;
begin
assign(input,'azshara.in');reset(input);
assign(output,'azshara.out');rewrite(output);
readln(m,n);
for i:=1 to n do
begin
 for j:=1 to m do
 begin
  read(s[i,j]);
  if s[i,j]=-1 then s[i,j]:=-maxint;
  end;
  readln;
  end;
 for i:=1 to n do
   for j:=1 to m do
    begin
     for k:=j-1 to j+1 do
    if f[i,j]<f[i-1,k]+s[i,j] then f[i,j]:=f[i-1,k]+s[i,j];
    end;
max:=f[n,1];
for i:=2 to m do
   if f[n,i]>max then max:=f[n,i];
if max<=0 then writeln('0')
          else writeln(max);
   end.