문제

Author: Goran Gašić

Consider the following sorting algorithm: reverse-sort(sequence a) while (a is not in nondecreasing order) partition a into the minimum number of slopes for every slope with length greater than one reverse(slope) A slope is defined as a decreasing consecutive subsequence of a. The reverse procedure will reverse the order of the elements in a slope. You are given a permutation of the first N natural numbers whose slopes all have even length when partitioned for the first time. Determine the total number of times reverse is called to reverse-sort the given permutation.

입력

The first line of input contains the positive integer N (2 ≤ N ≤ 100 000). The second line of input contains a permutation of the first N natural numbers that needs to be sorted.

출력

The only line of output must contain the number of times that reverse is called.

예제 입력 1

2
2 1

예제 출력 1

1

예제 입력 2

4
4 3 2 1

예제 출력 2

1

예제 입력 3

4
3 1 4 2

예제 출력 3

3