import sys, math, itertools, functools, collections
input = sys.stdin.readline

#https://stackoverflow.com/questions/4798654/modular-multiplicative-inverse-function-in-python
def egcd(a, b):
    if a == 0:
        return (b, 0, 1)
    else:
        g, y, x = egcd(b % a, a)
        return (g, x - (b // a) * y, y)

def minv(a, m):
    g, x, y = egcd(a, m)
    if g != 1:
        raise Exception('modular inverse does not exist')
    else:
        return x % m

def solve(testcase):
    n, w, d = map(int, input().split())
    a = [*map(int, input().split())]
    g = math.gcd(d, w)
    d2 = d // g
    w2 = w // g
    modinv = minv(d2, w2)
    def solve(x):
        # Find the min y such that (d*y)%w == x
        # d*y = a*w+x
        # d2*y = a*w2+x2
        if x%g:
            return 10**20
        x //= g
        # d * y === x (mod w)
        # y = x * modinv(d)
        #print('solve', x, (x * pow(d2, -1, w2)) % w2)
        return (x * modinv) % w2
    res = 0
    for i in range(n//2):
        x, y = a[i], a[~i]
        res += min(solve((x-y)%w), solve((y-x)%w))
    if res >= 10**20:
        res = 'IMPOSSIBLE'
    print('Case #'+str(testcase)+':', res)


for testcase in range(1, int(input())+1):
    solve(testcase)