#!/usr/bin/env python


def read():
    raw_input()
    return map(int, raw_input().split())


def work(vList):
    cnt = 0
    
    for i in range(len(vList) - 2):
        if vList[i] == vList[i + 1] or vList[i] == vList[i + 2] or \
           vList[i + 1] == vList[i + 2]:
            continue

        if not (sorted(vList[i : i + 3])[1] == vList[i    ] or \
                sorted(vList[i : i + 3])[1] == vList[i + 2]):
            continue
        
        cnt += 1

    print cnt


if __name__ == "__main__":
    work(read())