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seq_len = input()
sequence = [int(i) for i in input().split()]

i = 0
result1 = 0  # starting with lower
sequence2 = sequence.copy()
while i < len(sequence[:-1]):
    if i % 2 == 0:
        if sequence[i+1] <= sequence[i]:
            result1 += 1
            try:
                sequence[i+1] = sequence[i+2] + 1
            except IndexError:
                sequence[i+1] = sequence[i] + 1
    else:
        if sequence[i+1] >= sequence[i]:
            result1 += 1
            try:
                sequence[i+1] = sequence[i+2] - 1
            except IndexError:
                sequence[i+1] = sequence[i] - 1
    i += 1
    
i = 0
result2 = 0  # starting with greater
while i < len(sequence2[:-1]):
    if i % 2 == 0:
        if sequence2[i+1] >= sequence2[i]:
            result2 += 1
            try:
                sequence2[i+1] = sequence2[i+2] - 1
            except IndexError:
                sequence2[i+1] = sequence2[i] - 1
    else:
        if sequence2[i+1] <= sequence2[i]:
            result2 += 1
            try:
                sequence2[i+1] = sequence2[i+2] + 1
            except IndexError:
                sequence2[i+1] = sequence2[i] + 1
    i += 1

print(min(result1, result2))