program orase;
var f,g:text;
    m,n,i,j,ord,aux:longint;
    a,b:array[0..1000002]of longint;
begin
assign(f,'orase.in');
assign(g,'orase.out');
reset(f);
rewrite(g);
read(f,m,n);
for i:=1 to n do read(f,a[i],b[i]);
repeat
  ord:=0;
  for i:=1 to n-1 do
    if (a[i]>a[i+1])then
      begin
        aux:=a[i];
        a[i]:=a[i+1];
        a[i+1]:=aux;
        aux:=b[i];
        b[i]:=b[i+1];
        b[i+1]:=aux;
        ord:=1;
      end;
  until ord=0;
for i:=1 to n do
  for j:=i+1 to n do
    if (b[i]+b[j]+a[j]-a[i]>m)then
       m:=b[i]+b[j]+a[j]-a[i];
write(g,m);
closE(f);
close(g);
end.