1023B · Pair of Toys

Problem: 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.

Input Format: 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.

Output Format: 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.

Note: 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 $500000000000$.