class BIT:
	def query(self, m):
		res = 0
		while m > 0:
			res += self.bit[m]
			m -= m&(-m)
		return res
	
	def sum(self, a, b):
		return self.query(b)-self.query(a)
	
	def sumall(self):
		bitlen = self.bitlen-1
		return self.query(bitlen)
	
	def add(self, m, x):
		m += 1
		bitlen = len(self.bit)
		while m <= bitlen-1:
			self.bit[m] += x
			m += m&(-m)
		return
	
	def search(self, a):
		tmpsum = 0
		i = 0
		k = (self.bitlen-1).bit_length()
		while k >= 0:
			tmpk = 2**k
			if i+tmpk <= self.bitlen-1:
				if tmpsum+self.bit[i+tmpk] < a:
					tmpsum += self.bit[i+tmpk]
					i += tmpk
			k = k-1
		return i+1
	
	def __init__(self, a):
		self.bitlen = a
		self.bit = [0]*a

n, m = map(int,input().split())
p = list(map(int,input().split()))

for i in range(n):
	p[i] -= 1

bit = BIT(n+4)
ans = 0

for i in range(n):
	ans += bit.sum(p[i]+1, n+2);
	bit.add(p[i], 1);

k = (ans + m -1) // m
k *= m

if k % 2 == ans % 2:
	print(k)
	exit()

k += m

if k % 2 == ans % 2:
	print(k)
	exit()

print(-1)