fun main() {
    val (n, m) = readLongs()
    val p = readInts().map { it - 1 }
    val inv = inversionNumber(p)
    if (m % 2 == 0L && inv % 2 != 0L) {
        println(-1)
    } else {
        val s = (inv + m - 1) / m * m
        val k = s - inv
        if (k % 2 == 0L) {
            println(s)
        } else {
            val t = s + m
            val k2 = t - inv
            if (k2 % 2 == 0L) {
                println(t)
            } else {
                throw Exception()
            }
        }
    }
}

fun readLongs() = readln().split(" ").map { it.toLong() }
fun readInts() = readln().split(" ").map { it.toInt() }

class BIT(private val n: Int) {
    private val data = LongArray(n) { 0 }

    fun add(i: Int, x: Long) {
        var p = i + 1
        while (p <= n) {
            data[p - 1] += x
            p += p and -p
        }
    }

    fun sum(range: IntRange): Long {
        val l = range.first
        val r = range.last + 1
        return sum(r) - sum(l)
    }

    private fun sum(i: Int): Long {
        var r = i
        var s = 0L
        while(r > 0) {
            s += data[r - 1]
            r -= r and -r
        }
        return s
    }
}

fun inversionNumber(a: List<Int>): Long {
    val n = a.size
    val bit = BIT(n)
    var ans = 0L
    for (i in 0 until n) {
        ans += i - bit.sum(0..a[i])
        bit.add(a[i], 1)
    }
    return ans
}