n,m=map(int,input().split())
L=list(map(int,input().split()))


data = [0]*(n+1)
def add(i, x):
    while i <= n:
        data[i] += x
        i += i & -i
def get(i):
    s = 0
    while i:
        s += data[i]
        i -= i & -i
    return s

ans = 0
for a in L:
    add(a, 1)
    ans += a-get(a)

#ans+2x==km
if ans%2==1 and m%2==0:
  exit(print(-1))
k=0
if m<ans:
  k=ans//m
for i in range(k,k+10):
  if ans<=i*m and ans%2==i*m%2:
    exit(print(i*m))