#include <stdio.h>

using namespace std;

int main() {
    int n;
    scanf("%d", &n);
    int ans = 0;
    int i = 0;
    int a[n];
    for(i = 0; i < n; ++i)
    {
         scanf("%d", &a[i]);
    }
    for(i = 1; i < n -1; ++i)
    {
        if(a[i] == (a[i-1] + a[i+1])/2)
        {
             ans++;
        }
    }
    printf("%d", ans);
}