program Carnati;
var
   n,m,c,i,j,k,s,mn,i1,max1,j1,tt,ss,sst,mm,inc1,inc2:longint;
   t,p:array[0..2000] of longint;
   fl:text;
procedure qsort(in1,in2:longint);
var
   x,i,j:longint;
begin
   x:=t[(in1+in2) div 2];
   i:=in1;
   j:=in2;
   while i<j do
      begin
         while x>t[i] do
           i:=i+1;
         while x<t[j] do
           j:=j-1;
         if i<=j then
            begin
               k:=t[i];
               t[i]:=t[j];
               t[j]:=k;
               k:=p[i];
               p[i]:=p[j];
               p[j]:=k;
               i:=i+1;
               j:=j-1;
            end;
      end;
   if j>in1 then
      qsort(in1,j);
   if i<in2 then
     qsort(i,in2);
end;
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);

  qsort(1,n);

  s:=0;
  tt:=0;
  mm:=0;
  t[0]:=0;
  p[0]:=0;
  for i := 1 to n do
     begin
        mm:=0;
        for j := 1 to n do
           begin
              if p[j]>=p[i] then
                 i1:=p[i]
              else
                 i1:=0;

              ss:=i1+mm-(t[j]-t[j-1])*c;

              if ss<i1-c then
                 ss:=i1-c;

              if ss>max1 then
                 max1:=ss;
              mm:=ss;
           end;
     end;
   assign(fl,'carnati.out');
   rewrite(fl);
   writeln(fl,max1);
   close(fl);
end.