Exams

codefoces的一个作者参加了sat考试，现在给出他考试的科目数与他的总成绩，试问这位作者他最少有几个科目为2分（每科考试的分数为2到5分）

One day the Codeforces round author sat exams. He had $ n $ exams and he needed to get an integer from $ 2 $ to $ 5 $ for each exam. He will have to re-sit each failed exam, i.e. the exam that gets mark $ 2 $ . The author would need to spend too much time and effort to make the sum of his marks strictly more than $ k $ . That could have spoilt the Codeforces round. On the other hand, if the sum of his marks is strictly less than $ k $ , the author's mum won't be pleased at all. The Codeforces authors are very smart and they always get the mark they choose themselves. Also, the Codeforces authors just hate re-sitting exams. Help the author and find the minimum number of exams he will have to re-sit if he passes the exams in the way that makes the sum of marks for all $ n $ exams equal exactly $ k $ .

The single input line contains space-separated integers $ n $ and $ k $ ( $ 1<=n<=50 $ , $ 1<=k<=250 $ ) — the number of exams and the required sum of marks. It is guaranteed that there exists a way to pass $ n $ exams in the way that makes the sum of marks equal exactly $ k $ .

Print the single number — the minimum number of exams that the author will get a $ 2 $ for, considering that the sum of marks for all exams must equal $ k $ .

4 8

4

4 10

2

1 3

0

In the first sample the author has to get a $ 2 $ for all his exams. In the second sample he should get a $ 3 $ for two exams and a $ 2 $ for two more. In the third sample he should get a $ 3 $ for one exam.