var f,g:text;
    d,l:array[1..50000]of longint;
    n,m,i,j,max: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]);
max:=0;
for i:=1 to n-1 do
    for j:=i+1 to n do
        if i=j then
               else if l[i]+l[j]+abs(d[i]-d[j])>max then max:=l[i]+l[j]+abs(d[i]-d[j]);
writeln(g,max);
close(f);
close(g);
end.