var l,d:array[1..50000] of 0..1000000;
m,n,i,dmax,j,poz,t,aux:longint;
f,g:text;
k:boolean;
begin
assign(f,'orase.in');
assign(g,'orase.out');
reset(f);rewrite(g);
readln(f,m,n);
for i:=1 to n do
    read(f,d[i],l[i]);
t:=n;
repeat
      k:=true;
      for i:=1 to n-1 do
          if d[i]>d[i+1] then
             begin
             aux:=d[i];
             d[i]:=d[i+1];
             d[i+1]:=aux;
             aux:=l[i];
             l[i]:=l[i+1];
             l[i+1]:=aux;
             k:=false;
             end;
      t:=t-1;
until k=true;
j:=l[1]-d[1];
poz:=1;
for i:=2 to n do
    begin
    if l[i]+l[poz]+d[i]-d[poz]>dmax then
       dmax:=l[i]+l[poz]+d[i]-d[poz];
    if l[i]-d[i]>j then
       begin
       j:=l[i]-d[i];
       poz:=i;
       end;
    end;
write(g,dmax);
close(f);close(g);
end.