CF315A.Sereja and Bottles

Sereja and his friends went to a picnic. The guys had n soda bottles just for it. Sereja forgot the bottle opener as usual, so the guys had to come up with another way to open bottles.

Sereja knows that the i-th bottle is from brand a__i, besides, you can use it to open other bottles of brand b__i. You can use one bottle to open multiple other bottles. Sereja can open bottle with opened bottle or closed bottle.

Knowing this, Sereja wants to find out the number of bottles they've got that they won't be able to open in any way. Help him and find this number.

谢列亚和他的朋友们去野餐。他们为此准备了 $n$ 瓶汽水。谢列亚像往常一样忘记带开瓶器，因此大家不得不想出另一种开瓶的方法。

谢列亚知道第 $i$ 瓶汽水的品牌为 $a_i$，此外，它还可以用来开启其他品牌为 $b_i$ 的瓶子。一个瓶子可以用来开启多个其他瓶子。谢列亚可以用已开启的瓶子或未开启的瓶子来开启其他瓶子。

根据以上信息，谢列亚希望算出他们所拥有的、无论如何都无法开启的瓶子数量。请你帮助他求出这个数值。

The first line contains integer n (1 ≤ n ≤ 100) — the number of bottles. The next n lines contain the bottles' description. The i-th line contains two integers a__i, b__i (1 ≤ a__i, b__i ≤ 1000) — the description of the i-th bottle.

第一行包含一个整数 $n$（$1 \leq n \leq 100$）—— 瓶子的数量。接下来的 $n$ 行描述了各个瓶子。第 $i$ 行包含两个整数 $a_i$、$b_i$（$1 \leq a_i, b_i \leq 1000$）—— 第 $i$ 个瓶子的描述。

In a single line print a single integer — the answer to the problem.

在一行中输出一个整数——该问题的答案。