program jocul_Flip;
var
 n,m,i,j:byte;
 temp,s:longint;
 aduna:boolean;
 f,g:text;
begin
 assign(f,'flip.in');
 assign(g,'flip.out');
 reset(f);
 read(f,n);
 read(f,m);
 for i:=1 to n do
  begin
   aduna:=true;
   for j:=1 to m do
    begin
     read(f,temp);
     if aduna then
      if temp<0 then
       begin
        s:=s+temp*-1;
        aduna:=false;
       end
      else
       s:=s+temp;
    end;
  end;
 close(f);
 rewrite(g);
 write(g,s);
 close(g);
end.