program market;

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

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