program orase;
var f,g:text;
        v:array[1..50001] of longint;
        d:array[1..50001] of longint;
        m,n:longint;



procedure iofile;
var i: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],v[i]);
        close(f);
end;




procedure solve;
var dm,i,j:longint;
begin
        dm:=0;
        for i:=1 to n-1 do
                for j:=i+1 to n do
                        if v[i]+v[j]+abs(d[i]-d[j])>dm then
                        dm:=v[i]+v[j]+abs(d[i]-d[j]);
        writeln(g,dm);
        close(g);
end;



begin
        iofile;
        solve;
end.