var n,m,i,j,max,max2,nr1,nr2:longint;
    v,x:array[1..50010]of longint;

begin
assign(input,'orase.in');reset(input);
assign(output,'orase.out');rewrite(output);
readln(m,n);
for i:=1 to n do
        readln(v[i],x[i]);
if n<10000 then
        begin
        max:=0;
        for i:=1 to n-1 do
                for j:=i+1 to n do
                        begin
                        if x[i]+x[j]+abs(v[i]-v[j])>max then
                                max:=x[i]+x[j]+abs(v[i]-v[j]);
                        end;
        writeln(max);
        end
else
        begin
        max:=1;
        max2:=2;
        for i:=2 to n do
                if x[i]>x[max] then
                        begin
                        max2:=max;
                        max:=i;
                        end;
        nr1:=x[max]+x[max2]+abs(v[max]-v[max2]);
        max:=1;
        max2:=1;
        for i:=2 to n do
                begin
                if v[i]>v[max2] then
                        max2:=i;
                if v[i]<v[max] then
                        max:=i;
                end;
        nr2:=x[max]+x[max2]+abs(v[max]-v[max2]);
        if nr1>nr2 then
                writeln(nr1)
        else
                writeln(nr2);
        end;
close(input);close(output);
end.