import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################
# Binary Indexed Tree (Fenwick Tree)
# 0-indexed
class BIT:
    def __init__(self, n):
        self.n = n
        self.data = [0]*(n+1)
        self.el = [0]*(n+1)
    def sum(self, i):
        s = 0
        while i > 0:
            s += self.data[i]
            i -= i & -i
        return s
    def add(self, i, x):
        assert 0 <= i < self.n
        i = i+1
        self.el[i] += x
        while i <= self.n:
            self.data[i] += x
            i += i & -i
    def set(self, i, x):
        assert 0 <= i <= self.n
        self.add(i,x-self.get(i))
    def get(self, i, j=None):# j=Noneのときiを取得, [i, j)の和を取得
        if j is None:
            return self.el[i+1]
        return self.sum(j) - self.sum(i)
def tentou(ar):
    s = sorted(set(ar))
    d = {x:i for i, x in enumerate(s)}
    m = len(s)
    bit = BIT(m)
    ans = 0
    for i in range(n):
        ans += bit.get(d[ar[i]], m)
        bit.add(d[ar[i]], 1)
    return ans

n,m = na()
p = na()
z = tentou(p)

if z % 2 and m % 2 == 0:
    print(-1)
    exit()
else:
    x = (z + m -1)//m * m
    if (z-x)%2 == 0:
        print(x)
    else:
        print(x+m)