class FenwickTree:
    def __init__(self, size):
        self.n = size
        self.tree = [0] * (self.n + 1)
    
    def update(self, idx, delta):
        while idx <= self.n:
            self.tree[idx] += delta
            idx += idx & -idx
    
    def query(self, idx):
        res = 0
        while idx > 0:
            res += self.tree[idx]
            idx -= idx & -idx
        return res

def compute_inversion(p):
    max_val = len(p)
    ft = FenwickTree(max_val)
    inv_count = 0
    for x in reversed(p):
        inv_count += ft.query(x - 1)
        ft.update(x, 1)
    return inv_count

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

# Check if already sorted
is_sorted = all(p[i] == i + 1 for i in range(n))

if is_sorted:
    print(0)
else:
    s = compute_inversion(p)
    
    if m % 2 == 0:
        if s % 2 != 0:
            print(-1)
        else:
            t_min = (s + m - 1) // m
            print(t_min * m)
    else:
        t_min = (s + m - 1) // m
        required_parity = s % 2
        if (t_min % 2) == required_parity:
            print(t_min * m)
        else:
            print((t_min + 1) * m)