program factory;
var
  a:array[1..2,1..10000] of longint;
  i,n,m,min,k:longint;
Begin
  assign(input,'factory.in');
  assign(output,'factory.out');
  reset(input); rewrite(output);
  readln(n,m); min:=0; k:=1;
  for i:=1 to n do begin
    readln(a[1,i],a[2,i]);
    if a[1,i]<=a[1,k] then k:=i;
    min:=min+(a[1,k]*a[2,i]);
    a[1,k]:=a[1,k]+m;
  end;
  writeln(min);
  close(input); close(output);
End.