program cch(input,output);
var
 cost:int64;
 i,n,s,ci,yi,tot:longint;
 heap:array[1..10000] of int64;

procedure ins(x:longint);
var
 i,q:longint;
 tmp:int64;
begin
 for i:=1 to tot do heap[i]:=heap[i]+s;
 inc(tot);
 heap[tot]:=x;
 q:=tot;
 while (q>1)and(heap[q]<heap[q div 2]) do
  begin
   tmp:=heap[q];
   heap[q]:=heap[q div 2];
   heap[q div 2]:=tmp;
   q:=q div 2;
  end;
end;

begin
 assign(input,'factory.in');
 assign(output,'factory.out');
 reset(input);
 rewrite(output);
 readln(n,s);
 tot:=0;
 for i:=1 to n do
  begin
   readln(ci,yi);
   ins(ci);
   cost:=cost+heap[1]*yi;
  end;
 write(cost);
 close(input);
 close(output);
end.