program Carnati;
var
   n,m,c,i,j,k,s,mn,i1,j1,tt,ss,sst,mm:longint;
   t,p:array[0..2000] of longint;
   fl:text;
begin
  assign(fl,'carnati.in');
  reset(fl);
  readln(fl,n,c);
  for i := 1 to n do
     readln(fl,t[i],p[i]);
  close(fl);

  s:=0;
  tt:=0;
  mm:=0;
  t[0]:=t[1];
  p[0]:=p[1];
  for i := 1 to n do
    for j := i+1 to n do
       begin
          sst:=0;
          tt:=(t[j]-t[i]+1)*c;
          for i1 := i to j do
             begin
                 mm:=i1;
                 ss:=0;
                 for j1 := i to j do
                    if p[j1]>=p[mm] then
                       ss:=ss+p[mm];
                  if ss>sst then
                     sst:=ss;
             end;
          sst:=sst-tt;
          if sst>s then
             s:=sst;
       end;
   assign(fl,'carnati.out');
   rewrite(fl);
   writeln(fl,s);
   close(fl);
end.