# Google Code Jam 2023 Farewell Round B - Intruder Outsmarting
def gcd(a, b):
    if a % b == 0:
        return b
    else:
        return gcd(b, a % b)

# solve ax + by = gcd(a, b)
# return (gcd(a, b), x, y)
# require a>=0, b>=0
def extgcd(a, b):
    if b==0:
        return (a, 1, 0)
    else:
        t = extgcd(b, a%b)
        return (t[0], t[2], t[1] - (a//b)*t[2])


def mod_inverse(a, mod):
    r = extgcd(a, mod)
    return (r[1] % mod + mod) % mod

t = int(input())
for case in range(1, t + 1):
    w, n, d = map(int, input().split())
    x = [v - 1 for v in list(map(int, input().split()))]
    g = gcd(n, d)
    n //= g
    d //= g
    ans = 0
    for i in range(w//2):
        d1 = abs(x[i]-x[w-i-1])
        if d1 % g != 0:
            ans = "IMPOSSIBLE"
            break
        d1 //= g
        d2 = n-d1
        ans += min((d1*mod_inverse(d, n))%n, (d2*mod_inverse(d, n))%n)
    print("Case #{}: {}".format(case, ans))