def main():
    n = input()
    l = list(map(int, input().split()))
    ans = 0
    for i in range(len(l) - 2):
        a = l[i:i + 3]
        if len(set(a)) == 3 and sorted(l[i:i+3])[1] != l[i+1]:
            ans += 1
    print(ans)

    
if __name__ == '__main__':
    main()