R=range

def rec_str(a):
	if not isinstance(a,list):return str(a)
	return "".join(["[",", ".join(rec_str(x)for x in a),"]"])
class BIT:
	def __init__(self,x):
		if isinstance(x,int):
			self.N=x
			self.F=[0]*(x+1)
		else:
			self.N=len(x)
			self.F=[0]*(self.N+1)
			for j in R(1,self.N+1):
				i=j-1
				self.F[j]=x[i]
				k=j-(j&-j)
				while i>k:
					self.F[j]+=self.F[i]
					i-=i&-i
	def Add(self,i,u):
		i+=1
		while i<=self.N:
			self.F[i]+=u
			i+=i&-i
	def Set(self,i,u):
		self.Add(i,u-self.Get(i))
	def Get(self,i):
		return self.IntervalSum(i,i)
	def InitialSegmentSum(self,r):
		assert(r>-2)
		a=0
		i=min(r+1,self.N)
		while i:
			a+=self.F[i]
			i-=i&-i
		return a
	def IntervalSum(self,l,r):
		return self.InitialSegmentSum(r)-self.InitialSegmentSum(l-1)
	def list(self):
		return[self.Get(i)for i in R(self.N)]
	def __str__(self):
		return rec_str(self.list())
	def BinarySearch(self,u):
		j=s=n=0
		p=1<<17 #131072
		while p:
			k=j|p
			p>>=1
			if k<=N:
				n+=F[k]
				if n<u:s,j=n,k
				else:n=s
		return j

J=lambda:map(int,input().split())
N,M=J()
P=list(J())
Q=[0]*N
for i in R(N):P[i]-=1;Q[P[i]]=i
count=BIT(N)
n=0
for i in R(N-1,-1,-1):
	count.Add(Q[i],1)
	n+=count.IntervalSum(0,Q[i]-1)
a=(n+M-1)//M*M
if(a-n)&1:
	if M&1:a+=M
	else:a=-1
print(a)