var f,g:text;
    d,l:array[1..50010] of longint;
    m,n,i,max,j:longint;
begin
     assign(f,'orase.in'); reset(f);
     assign(g,'orase.out'); rewrite(g);
     readln(f,m,n);
     for i:=1 to n do readln(f,d[i],l[i]);
     for i:=1 to n-1 do
          for j:=i+1 to n do
               if max<abs(d[i]-d[j])+l[i]+l[j] then max:=abs(d[i]-d[j])+l[i]+l[j];
     writeln(g,max);
     close(g);
end.