n, k = gets.split.map(&:to_i)
a = gets.split.map(&:to_i)
ans = 0
if n > k
    loop{
        c = 0
        (n - k).times{|i|
            if a[i] > a[i + k]
                tmp = a[i]
                a[i] = a[i + k]
                a[i + k] = tmp
                c += 1
            end
            }
        if c == 0
            break
        end
        ans += c
        }
end
if a == a.sort
    p ans
else
    p -1
end