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if __name__ == "__main__":
    n: str = input()
    chain: str = input()

    chain: list = [int(x) for x in chain.split(" ")]

    result = 0
    result2 = 0
    for i in range(1, len(chain)-1, 2):
        if (chain[i] > chain[i-1]) or (chain[i] > chain[i+1]):
            chain[i] = min(chain[i-1], chain[i+1]) - 1
            result +=1

        elif (chain[i] < chain[i-1]) or (chain[i] < chain[i+1]):
            chain[i] = max(chain[i-1], chain[i+1]) + 1
            result2 +=1


    print(min(result, result2))