n = int(input())
points = [[int(i) for i in input().split()] for _ in range(n+1)]

left = [
    ((-1, 0), (0, -1)),
    ((0, -1), (1, 0)),
    ((1, 0), (0, 1)),
    ((0, 1), (-1, 0))
]

right = [(p2, p1) for p1, p2 in left]

dangerous_turns = 0

def signe(n):
    if n == 0: return 0
    if n > 0: return 1
    return -1

def normalized(x1, y1, x2, y2, x3, y3):
    return ((signe(x2-x1), signe(y2-y1)), (signe(x3-x2), signe(y3-y2)))

def danger(initial_turn, p1, p2, p3):
    norm = normalized(*p1, *p2, *p3)
    if (initial_turn and norm in left) or (not initial_turn and norm in right):
        return False
    return True


initial_turn = normalized(*points[-2], *points[0], *points[1]) in left
#print("initial:", normalized(*points[-2], *points[0], *points[1]), initial_turn)
for i in range(1, n):
    if danger(initial_turn, points[i-1], points[i], points[i+1]):
        dangerous_turns += 1

print(dangerous_turns)