Problem

N idols, numbered from 1 to n in order, are lined up in a row.
Idle i can transmit information to idle i-1 and idle i + 1 in a unit time. However, idol 1 can transmit information only to idol 2, and idol n can transmit information only to idol n-1.

At time 0, m idols with numbers a1, a2, ..., am have secret information. Find the minimum amount of time all idols can get confidential information.

Constraints

• 2 ≤ n ≤ 105
• 1 ≤ m ≤ n
• 1 ≤ ai ≤ n
• All ai values ​​are different
• ai are given in ascending order

Input

The input is given in the following format.

n m
a1 a2 ... am

Two integers n and m are given on the first line, separated by blanks.
On the second line, m integers a1, a2, ..., am are given, separated by blanks.

Output

Outputs the minimum time that information is transmitted to all idles on one line.

Examples

Input

3 2
1 3

Output

1

Input

10 3
2 5 7

Output

3

Input

10 5
2 5 6 8 10

Output

1

Input

100000 1
1

Output

99999

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