program orase;
var f,g:text;
    v:array[1..1000000]of 0..1000000;
    m,n,a,b,i,max,j:longint;
begin
assign(f,'orase.in');
assign(g,'orase.out');
reset(f);
rewrite(g);
read(f,m,n);
for i:=1 to n do
  begin
    read(f,a,b);
    if (v[a]<b)then v[a]:=b;
    if (a>max)then max:=a;
  end;
m:=0;
for i:=1 to max do
  if (v[i]>0)then
    for j:=i to max do
      if (v[j]>0)then
        if (v[i]+v[j]+j-i>m)then m:=v[i]+v[j]+j-i;
write(g,m);
closE(f);
close(g);
end.