# 往届 ACM 队训练题 (参考)

1033. ZigZag

A sequence of numbers is called a zig-zag sequence if the differences between successive numbers strictly alternate between positive and negative. The first difference (if one exists) may be either positive or negative. A sequence with fewer than two elements is trivially a zig-zag sequence.

For example, 1,7,4,9,2,5 is a zig-zag sequence because the differences (6,-3,5,-7,3) are alternately positive and negative. In contrast, 1,4,7,2,5 and 1,7,4,5,5 are not zig-zag sequences, the first because its first two differences are positive and the second because its last difference is zero.

### 输入格式

Given a sequence of integers, sequence, return the length of the longest subsequence of sequence that is a zig-zag sequence. A subsequence is obtained by deleting some number of elements (possibly zero) from the original sequence, leaving the remaining elements in their original order.

• sequence contains between 1 and 50 elements, inclusive.
• Each element of sequence is between 1 and 1000, inclusive.

### 输出格式

output the length of the longest subsequence of sequence that is a zig-zag sequence.

### 样例

Input
10
1 17 5 10 13 15 10 5 16 8

Output
7

Input
6
1 7 4 9 2 5

Output
6