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#!/usr/bin/env python3
# coding=utf-8

# Copyright (C) 2022  Paweł Widera
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details:
# http://www.gnu.org/licenses/gpl.html

def up_first(numbers):
	# add a guard number (end on a rise)
	numbers.append(numbers[-1] - 1)

	total = 0
	for i in range(1, n, 2):
		if not (numbers[i - 1] < numbers[i] > numbers[i + 1]):
			if (i + 2 < n and numbers[i - 1] < numbers[i] <= numbers[i + 1]):
				numbers[i + 1] = min(numbers[i], numbers[i + 2]) - 1
			else:
				numbers[i] = max(numbers[i - 1], numbers[i + 1]) + 1
			total += 1
	return total


def down_first(numbers):
	# add a guard number (end on a decline)
	numbers.append(numbers[-1] + 1)

	total = 0
	for i in range(1, n, 2):
		if not (numbers[i - 1] > numbers[i] < numbers[i + 1]):
			if (i + 2 < n and numbers[i - 1] > numbers[i] >= numbers[i + 1]):
				numbers[i + 1] = max(numbers[i], numbers[i + 2]) + 1
			else:
				numbers[i] = min(numbers[i - 1], numbers[i + 1]) - 1
			total += 1
	return total


n = int(input())
numbers = [int(x) for x in input().split()]

# check two variants of a first change
up_count = up_first(numbers[:])
down_count = down_first(numbers)

print(min(up_count, down_count))