You are given a permutation p_1,p_2,...,p_N consisting of 1,2,..,N.
You can perform the following operation any number of times (possibly zero):
Operation: Swap two adjacent elements in the permutation.
You want to have p_i ≠ i for all 1≤i≤N.
Find the minimum required number of operations to achieve this.

-----Constraints-----
- 2≤N≤10^5
- p_1,p_2,..,p_N is a permutation of 1,2,..,N.

-----Input-----
The input is given from Standard Input in the following format:
N
p_1 p_2 .. p_N

-----Output-----
Print the minimum required number of operations

-----Sample Input-----
5
1 4 3 5 2

-----Sample Output-----
2

Swap 1 and 4, then swap 1 and 3. p is now 4,3,1,5,2 and satisfies the condition.
This is the minimum possible number, so the answer is 2.

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