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import sys

a = ""
b = ""

a = input()
b = input()

n = int(a)
tmp = b.split(" ")
ciag = []
for a in tmp:
	ciag.append(int(a))

zmian = 0
gora = False
if ciag[0] == ciag[1]:
	if ciag[1] == ciag[2]:
		ciag[1] += 1
		gora = False
		zmian += 1
	elif ciag[1] > ciag[2]:
		ciag[0] -= 1
		gora = False
		zmian += 1
	else:
		ciag[0] += 1
		gora = True
		zmian += 1
elif ciag[0] > ciag[1]:
	if ciag[1] == ciag[2]:
		ciag[2] += 1
		gora = True
		zmian += 1
	elif ciag[1] > ciag[2]:
		ciag[2] += (abs(ciag[1]) + abs(ciag[2]) + 1)
		gora = False
		zmian += 1
	else:
		gora = True
elif ciag[0] < ciag[1]:
	if ciag[1] == ciag[2]:
		ciag[2] -= 1
		gora = False
		zmian += 1
	elif ciag[1] > ciag[2]:
		gora = False
	else:
		ciag[2] -= (abs(ciag[1]) + abs(ciag[2]) + 1)
		gora = False
		zmian += 1


if n > 3:
	i = 2
	while i < n - 1:
		if gora and ciag[i] > ciag[i+1]:
			gora = False
		elif gora and ciag[i] <= ciag[i+1]:
			ciag[i+1] -= (abs(ciag[i]) + abs(ciag[i+1]) + 1)
			gora = False
			zmian += 1
		elif not gora and ciag[i] >= ciag[i+1]:
			ciag[i+1] += (abs(ciag[i]) + abs(ciag[i+1]) + 1)
			gora = True
			zmian += 1
		else:
			gora = True
		i += 1

print(zmian)