var f,g:text;
    d,l:array[1..50010] of longint;
    m,n,i,max1,max2,x,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 do
     begin
          x:=l[i]+d[i];
          if max1<x then max1:=x;
     end;
     for i:=1 to n do
     begin
          x:=l[i]-d[i];
          if max2<x then max2:=x;
     end;
     writeln(g,max1+max2);
     close(g);
end.