def f(x):
    if x % P == 0:
        x //= P
    return x

N, M, P, *A = map(int, open(0).read().split())

max_A = max(A)
t = max(map(f, A))

if max_A > M:
    print(1)
    exit()

if t == 1:
    print(-1)
    exit()

x = 1
result = 0
while True:
    result += 1
    if x * max_A > M:
        break
    x *= t
print(result)