program market;

var
   n, c, i, j, k, max, min, pr, tmax, l, tmin: longint;
   t, p: array[1..2010] of longint;
   f: text;

begin
   assign(f, 'carnati.in');
   reset(F);
   max:=0;
   readln(f, n, c);
   for i:= 1 to n do
      readln(f, t[i], p[i]);
{  for i:= 1 to n do
      for j:= i to n do
         if t[i]>t[j] then
            begin
               tmp:=t[i];
               t[i]:=t[j];
               t[j]:=tmp;
               tmp:=p[i];
               p[i]:=p[j];
               p[j]:=tmp;
            end;}
   for i:=1 to n do
      for j:= i to n do
         for k:=i to j do
            begin
               min:=p[k];
               tmin:=t[k];
               tmax:=t[k];
               pr:=0;
               for l:=i to j do
                  if min<=p[l] then
                     begin
                        pr:=pr+1;
                        if t[l]<tmin then
                           tmin:=t[l];
                        if t[l]>tmax then
                           tmax:=t[l];
                     end;
               pr:=min*pr-c*(tmax-tmin+1);
               if max<pr then
                  max:=pr;
         end;
   close(F);
   assign(f, 'carnati.out');
   rewrite(F);
   writeln(f, max);
   close(F);
end.