class Bit:
    def __init__(self, n):
        self.size = n
        self.tree = [0] * (n + 1)
  
    def sum(self, i):
        s = 0
        while i > 0:
            s += self.tree[i]
            i -= i & -i
        return s
  
    def add(self, i, x):
        while i <= self.size:
            self.tree[i] += x
            i += i & -i
 
N, M = map(int, input().split())
P = list(map(int, input().split()))
bit = Bit(N)
cnt = 0
for i, p in enumerate(P):
    cnt += i - bit.sum(p)
    bit.add(p, 1)
 
k = 0
while M*k<cnt:
    k+=1

if (M*k-cnt)%2==0:
    ans = M*k
    print(ans)
else:
    k+=1
    if (M*k-cnt)%2==0:
        ans = M*k
        print(ans)
    else:
        print(-1)