import sys
import math
import bisect
from heapq import heapify, heappop, heappush
from collections import deque, defaultdict, Counter
from functools import lru_cache
from itertools import accumulate, combinations, permutations, product

sys.set_int_max_str_digits(10 ** 6)
sys.setrecursionlimit(1000000)
MOD = 10 ** 9 + 7
MOD99 = 998244353

input = lambda: sys.stdin.readline().strip()
NI = lambda: int(input())
NMI = lambda: map(int, input().split())
NLI = lambda: list(NMI())
SI = lambda: input()
SMI = lambda: input().split()
SLI = lambda: list(SMI())
EI = lambda m: [NLI() for _ in range(m)]


def compress(S):
    """ 座標圧縮 """

    S = set(S)
    zipped, unzipped = {}, {}
    for i, a in enumerate(sorted(S), start=1):
        zipped[a] = i
        unzipped[i] = a
    return zipped, unzipped


class BIT():
    def __init__(self, n):
        """
        1-index
        sum -> i番目までの和
        add -> i番目にxを足す
        :param n:
        """
        self.n = n
        self.data = [0] * (n + 1)
        self.each = [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):
        self.each[i] += x
        while i <= self.n:
            self.data[i] += x
            i += i & -i

    def __repr__(self):
        return str(self.each)


def inversion_num(A):
    """
    リストAの転倒数を座標圧縮ありで求める。要BIT, compress
    """
    N = len(A)
    Z, _ = compress(A)
    tree = BIT(N)
    res = 0
    for a in A:
        za = Z[a]
        res += tree.sum(za)
        tree.add(za, 1)
    return N * (N - 1) // 2 - res


def main():
    N, M = NMI()
    P = NLI()
    x = inversion_num(P)
    ans = (x+M-1)//M * M
    if (ans-x) % 2:
        if M % 2:
            print(ans + M)
        else:
            print(-1)
    else:
        print(ans)


if __name__ == "__main__":
    main()