import math

def exgcd(m, n):
    if n>0:
        y,x,d = exgcd(n, m%n)
        return x, y-m//n*x, d
    else:
        return 1, 0, m

T = int(input())
for t in range(T):
    W, N, D = map(int, input().split())
    X = [int(x)-1 for x in input().split()]
    ans = 0
    oo = 10**20
    for i in range(W//2):
        d = X[i]-X[-i-1]
        g = math.gcd(N, D)
        if d%g!=0:
            ans += oo
        else:
            d //= g
            #a = d*pow(D//g, -1, N//g)%(N//g)
            x, y, _ = exgcd(D//g, N//g)
            a = d*x%(N//g)
            ans += min(a, (N//g)-a)
    if ans>=oo:
        ans = "IMPOSSIBLE"
    print(f"Case #{t+1}: {ans}")