program azshara;
var
  table:array[0..2,1..1000]of longint;
  w,l,i,j,p1,p2,p,max:longint;
begin
  assign(input,'azshara.in');
  assign(output,'azshara.out');
  reset(input);
  rewrite(output);
  readln(w,l);
  p1:=0;
  p2:=1;
  fillchar(table,sizeof(table),0);
  for i:=1 to l do
  begin
    for j:=1 to w do
      read(table[2,j]);
    readln;
    for j:=1 to w do
    begin
      p:=0;
      if j-1>0 then if (table[p2,j-1]+table[2,j]>table[p1,j])and(table[p2,j-1]<>-1) then begin
        table[p1,j]:=table[p2,j-1]+table[2,j];
        p:=1;
      end;
      if (table[p2,j]+table[2,j]>table[p1,j])and(table[p2,j]<>-1) then begin
        table[p1,j]:=table[p2,j]+table[2,j];
        p:=1;
      end;
      if j+1<=w then if (table[p2,j+1]+table[2,j]>table[p1,j])and(table[p2,j+1]<>-1) then begin
        table[p1,j]:=table[p2,j+1]+table[2,j];
        p:=1;
      end;
      if p=0 then table[p1,j]:=-1;
    end;
    p1:=p2;
    p2:=abs(p1-1);
  end;
  max:=0;
  for i:=1 to w do
    if table[p2,i]>max then max:=table[p2,i];
  writeln(max);
  close(input);
  close(output);
end.