CF1023B.Pair of Toys

Tanechka is shopping in the toy shop. There are exactly $n$ toys in the shop for sale, the cost of the $i$-th toy is $i$ burles. She wants to choose two toys in such a way that their total cost is $k$ burles. How many ways to do that does she have?

Each toy appears in the shop exactly once. Pairs $(a, b)$ and $(b, a)$ are considered equal. Pairs $(a, b)$, where $a=b$, are not allowed.

塔涅奇卡正在玩具店购物。店里恰好有 $n$ 个待售玩具，第 $i$ 个玩具的价格为 $i$ 卢布。她希望挑选两个玩具，使得它们的总价格恰好为 $k$ 卢布。她有多少种选择方式？

每个玩具在店里恰好出现一次。对 $(a, b)$ 和 $(b, a)$ 被视为同一种方案。不允许选择同一个玩具两次（即不允许 $a = b$）。

The first line of the input contains two integers $n$, $k$ ($1 \le n, k \le 10^{14}$) — the number of toys and the expected total cost of the pair of toys.

输入的第一行包含两个整数 $n$、$k$（$1 \le n, k \le 10^{14}$）—— 分别表示玩具的数量以及一对玩具的期望总成本。

Print the number of ways to choose the pair of toys satisfying the condition above. Print 0, if Tanechka can choose no pair of toys in such a way that their total cost is $k$ burles.

输出满足上述条件的玩具对的选择方案数。如果塔涅奇卡无法选择任何一对玩具，使其总价格恰好为 $k$ 卢布，则输出 0。

In the first example Tanechka can choose the pair of toys ($1, 4$) or the pair of toys ($2, 3$).

In the second example Tanechka can choose only the pair of toys ($7, 8$).

In the third example choosing any pair of toys will lead to the total cost less than $20$. So the answer is 0.

In the fourth example she can choose the following pairs: $(1, 1000000000000)$, $(2, 999999999999)$, $(3, 999999999998)$, ..., $(500000000000, 500000000001)$. The number of such pairs is exactly $500000000000$.

在第一个例子中，塔涅奇卡可以选择玩具对 $(1, 4)$ 或玩具对 $(2, 3)$。

在第二个例子中，塔涅奇卡只能选择玩具对 $(7, 8)$。

在第三个例子中，任意选择一对玩具，其总花费均小于 $20$。因此答案为 $0$。

在第四个例子中，她可以选择以下这些对：$(1, 1000000000000)$、$(2, 999999999999)$、$(3, 999999999998)$、……、$(500000000000, 500000000001)$。这样的对的数量恰好为 $500000000000$。