bit = [0] * 220000


def add(x):
    x += 1
    while x < len(bit):
        bit[x] += 1
        x += x & -x


def getSum(x):
    x += 1
    ans = 0
    while x > 0:
        ans += bit[x]
        x -= x & -x

    return ans


N, M = map(int, input().split())
P = list(map(int, input().split()))

r = 0

for p in P[::-1]:
    r += getSum(p)
    add(p)


ans = r // M * M

for _ in range(6):
    if ans >= r and ans % 2 == r % 2:
        print(ans)
        exit()
    ans += M

print(-1)