class BIT:
    def __init__(self,n):
        self.n = n; self.k = [0]*(n+1)
    def a(self,i,x):
        while i<=self.n: self.k[i] += x; i += i&-i
    def s(self,i):
        t = 0
        while i>0: t += self.k[i]; i -= i&-i
        return t
n,m = map(int,input().split())
p = list(map(int,input().split()))
bit = BIT(n); x = 0
for i,v in enumerate(p): x += i-bit.s(v); bit.a(v,1)
if m%2==0 and m%2!=x%2: print(-1)
else: y = -(-x//m)*m; print(y if y%2==x%2 else y+m)