def bitadd(a,w,bit): #aにwを加える(1-origin)
 
    x = a
    while x <= (len(bit)-1):
        bit[x] += w
        x += x & (-1 * x)
 
def bitsum(a,bit): #ind 1～aまでの和を求める
 
    ret = 0
    x = a
    while x > 0:
        ret += bit[x]
        x -= x & (-1 * x)
    return ret


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

BIT = [0] * (N+1)
a = list(map(int,input().split()))

ans = 0
for i in range(N):

    ans += i - bitsum(a[i],BIT)
    bitadd(a[i],1,BIT)

if ans % M == 0:
    print (ans)
elif M % 2 == 0 and ans % 2 == 1:
    print (-1)
else:
    
    now = ans // M * M
    while (True):

        if now >= ans and (now-ans) % 2 == 0:
            print (now)
            break

        now += M