A8650. Exams

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 $k$ .

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.