program comutari;
var n,j,m,n:integer;
    sp,sn,s:integer;
    f,g:text;
begin
assign(f,'flip.in');
reset(f);
assigh(g,'flip.out');
rewrite(g);
read(f,n);
read(f,m);
  For i:=1 to n do
     for J:=1 to m do
        begin
          read(f,a[i,j]);
        end;
   I:=1;
   sp:=0;
   s:=0;
   sn:=0;
       While i<=n do
          for j:=1 to m do
            begin
               If a[i,j]>0 then
               begin
                  sp:=sp+a[i,j];
               end
               else
               begin
                  sn:=sn-a[i,j];
                  end;
               If sp<sn then
                 begin
                 a[i,j]:=-a[i,j];
                 end;
               i:=i+1;
            end;
       For i:=1 to n do
          FOr j:=1 to m do
          begin
            s:=s+a[i,j];
          end;
          write(g,s);

close(f);
close(g);

end.