program pb002;
type vector=array [1..256] of longint;
var f,g:text;
    u,k,i,j,s:longint;
    n,m:1..16;
    x:-1000000000..1000000000;
    a:vector;

procedure poz(li,ls:longint; var k:longint; var a:vector);
var i,j,c,i1,j1:longint;
begin
i1:=0;
j1:=-1;
i:=li;
j:=ls;
while i<j do begin
        if a[i]>a[j] then begin
                c:=a[j];
                a[j]:=a[i];
                a[i]:=c;
                c:=i1;
                i1:=-j1;
                j1:=-c;
                end;
        i:=i+i1;
        j:=j+j1;
        end;
k:=i;
end;

procedure quick(li,ls:integer);
begin
if li<ls then begin
        poz(li,ls,k,a);
        quick(li,k-1);
        quick(k+1,ls);
        end
end;

begin
assign(f,'flip.in');
assign(g,'flip.out');
reset(f);
rewrite(g);
readln(f,n,m);
u:=0; s:=0;
for i:=0 to n-1 do begin
for j:=1 to m do begin
                 read(f,x);
                 a[(i*m)+j]:=abs(x);
                 s:=s+abs(x);
                 if x<0 then u:=u+1;
                 end;
readln(f);
end;
quick(1,m*n);
for i:=1 to u do s:=s-(2*a[i]);
write(g,s);
close(f);
close(g);
end.