class BIT:
    def __init__(self, n):
        self.size = n
        self.tree = [0]*(n+1)
 
    def build(self, list):
        self.tree[1:] = list.copy()
        for i in range(self.size+1):
            j = i + (i & (-i))
            if j < self.size+1:
                self.tree[j] += self.tree[i]

    def sum(self, i):
        # [0, i) の要素の総和を返す
        s = 0
        while i>0:
            s += self.tree[i]
            i -= i & -i
        return s
    # 0 index を 1 index に変更  転倒数を求めるなら1を足していく
    def add(self, i, x):
        i += 1
        while i <= self.size:
            self.tree[i] += x
            i += i & -i

    # 総和がx以上になる位置のindex をbinary search
    def bsearch(self,x):
        le = 0
        ri = 1<<(self.size.bit_length()-1)
        while ri > 0:
            if le+ri <= self.size and self.tree[le+ri]<x:
                x -= self.tree[le+ri]
                le += ri
            ri >>= 1
        return le+1
        
        
n,m = map(int,input().split())
P = [int(x)-1 for x in input().split()]

num = 0
bit = BIT(n+1)

for i,p in enumerate(P):
	num += i-bit.sum(p)
	bit.add(p,1)


if num%m == 0:
	print(num)
elif num%2 != m%2 and num%2:
	print(-1)
else:
	dif = m - num%m
	print(num+dif)