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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;

namespace B
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            string[] In = Console.ReadLine().Split(' ');
            int[] P = new int[n];
            for(int i=0; i < n; i++)
            {
                P[i] = int.Parse(In[i]);
            }
            int cnt = 0;
            for(int i = 1; i < n - 1; i++)
            {
                if (P[i - 1] < P[i] && P[i] < P[i + 1]) cnt++;
                if (P[i - 1] > P[i] && P[i] > P[i + 1]) cnt++;
            }
            Console.WriteLine(cnt);
        }
    }
}