import sys
input = sys.stdin.readline
class BIT:
    def __init__(self, n):
        self.size = n
        self.bit = [0] * (n + 1)
    def sum(self, i): # [0,i]
        i+=1
        s = 0
        while i > 0:
            s += self.bit[i]
            i -= i & -i
        return s
    def sumrange(self, i, j): # [i,j)
        return self.sum(j-1) - self.sum(i-1)
    def add(self, i, x):
        if x==0:return
        i+=1
        while i <= self.size:
            self.bit[i] += x
            i += i & -i

def inversion_number(A):
    bit = BIT(max(A)+1)
    ans = 0
    for i, a in enumerate(A):
        bit.add(a, 1)
        ans += i + 1 - bit.sum(a)
    return ans

N,M=map(int,input().split())
P=list(map(int,input().split()))
n = inversion_number(P)
# n + 偶数回 == M*a となる最小のMa
if n%2==1 and M%2==0:
    print(-1)
else:
    m = (n+M-1)//M*M
    if n%2 != m%2:
        m+=M
    print(m)