How many ways are there to choose two distinct positive integers totaling N, disregarding the order?
if __name__ == '__main__':
	n = int(input())
	if n % 2 == 0:
		print(n/2 - 1)
	else:
		print((n-1)/2)

if __name__ == '__main__':
    n = int(input())
    if n % 2 == 0:
        print(int(n/2 - 1))
    else:
        print(int((n-1)/2))