Problem:
Score : 200 points

Problem Statement

We have a permutation p = {p_1,\ p_2,\ ...,\ p_n} of {1,\ 2,\ ...,\ n}.

Print the number of elements p_i (1 < i < n) that satisfy the following condition:

p_i is the second smallest number among the three numbers p_{i - 1}, p_i, and p_{i + 1}.

Constraints

All values in input are integers.

3 \leq n \leq 20

p is a permutation of {1,\ 2,\ ...,\ n}.

Input

Input is given from Standard Input in the following format:

n
p_1 p_2 ... p_n

Output

Print the number of elements p_i (1 < i < n) that satisfy the condition.

Sample Input 1

5
1 3 5 4 2

Sample Output 1

2

p_2 = 3 is the second smallest number among p_1 = 1, p_2 = 3, and p_3 = 5. Also, p_4 = 4 is the second smallest number among p_3 = 5, p_4 = 4, and p_5 = 2. These two elements satisfy the condition.

Sample Input 2

9
9 6 3 2 5 8 7 4 1

Sample Output 2

5

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include "bits/stdc++.h"

using namespace std;

//using Matrix = vector< vector<int> >;
typedef long long ll;
typedef pair<int, int> P;
const int INF = 1 << 30;
int dx[5] = {0, 0, 1, -1, 0}, dy[5] = {1, -1, 0, 0, 0};
bool cmp(P a, P b) { return a.second < b.second; } //End sort (Interval scheduling problem)

int n;
int p[30];

int main(void){
	cin>>n;
	for (int i = 0; i < n; i++) cin>>p[i];

	int ans = 0;
	for (int i = 1; i + 1 < n; i++) {
		if ((p[i - 1] < p[i] && p[i] < p[i + 1]) || (p[i + 1] < p[i] && p[i] < p[i - 1]))
			ans++;
	}

	cout<< ans <<endl;
	
	return 0;
}