fun main() {
    val t = readLine()!!.toInt()
    for (i in 1..t) {
        print("Case #$i: ")
        solve()
    }
}

fun solve() {
    val (w, n, d) = readLine()!!.split(' ').map(String::toInt)
    val x = readLine()!!.split(' ').map { it.toLong() - 1 }.toLongArray()

    val (xx, yy, gcd) = gcd(d.toLong(), n.toLong())

    fun calc(ab: Long): Long {
        // dx + yn = ab
        val dd = ab / gcd
        var xxx = (xx * dd) % (n / gcd)
        if (xxx < 0) xxx += n / gcd
        return minOf(xxx, -(xxx - n / gcd))
    }

    val ab = (0 until w / 2).map { x[it] - x[w - it - 1] }
    if (ab.any { it % gcd != 0L }) println("IMPOSSIBLE") else println(ab.sumOf { calc(it) })
}

fun gcd(a: Long, b: Long): Triple<Long, Long, Long> {
    var x0 = 1L
    var y0 = 0L
    var x1 = 0L
    var y1 = 1L
    var aa = a
    var bb = b
    while (bb != 0L) {
        val d = aa / bb
        val nb = aa - d * bb
        val nx1 = x0 - d * x1
        val ny1 = y0 - d * y1
        aa = bb
        x0 = x1
        y0 = y1
        bb = nb
        x1 = nx1
        y1 = ny1
    }
    return Triple(x0, y0, aa)
}