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Problems > Local or Global

Local or Global

Difficulty: 1300 4000 ms 128 MB

math implementation

A permutation's LOCAL inversions are adjacent ones (ai > ai+1); GLOBAL inversions are any i < j with ai > aj. Print YES if every global inversion is local (the two counts coincide).

Input: the first line has N; the second line has a permutation of 1..N.

Output: YES or NO.

Constraints:

  • 1 ≤ N ≤ 105

Sample Tests

Input 1
3
1 3 2
Output 1
YES
Input 2
3
3 1 2
Output 2
NO

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