type oras=record
 d,l:word;
 end;
var a:array[1..50000] of oras;
    aux:oras;
    n,m,max,s,i,j:longint;
    f,g:text;
begin
 assign(f,'orase.in');
 reset(f);
 read(f,m);
 readln(f,n);
 for i:=1 to n do
  with a[i] do begin
   read(f,d);
   readln(f,l);
   end;
 close(f);
 for i:=1 to n-1 do
  if a[i].l<a[i+1].l then begin
   aux:=a[i];
   a[i]:=a[i+1];
   a[i+1]:=aux;
   end;
 max:=0;
 for j:=1 to 2 do
 for i:=j+1 to n do begin
  s:=0;
  if a[j].d>a[i].d then s:=a[j].d-a[i].d
   else if a[j].d<a[i].d then s:=a[i].d-a[j].d
    else s:=a[j].d;
  s:=s+a[j].l+a[i].l;
  if s>max then max:=s;
  end;
 assign(g,'orase.out');
 rewrite(g);
 write(g,max);
close(g);
end.