program orase;
var a,d:array[1..50000] of longint;
n,i,j,m,l,s:longint;
f,g:text;
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],d[i]);
  l:=0;
  for i:=1 to n-1 do
   for j:=i+1 to n do
     begin
       if a[i]>a[j] then s:=a[i]-a[j]+d[i]+d[j] else s:=a[j]-a[i]+d[i]+d[j];
       if s>l then l:=s;
     end;
  writeln(g,l);
  close(f); close(g);
end.