n,m,p = map(int,input().split())
a = list(map(int,input().split()))
m_a = max(a)

s = -1
for x in a:
    if x//p == 0:
        pass
    else:
        s = max(s, x)
if s == -1:
    if m_a > m:
        print(1)
    else:
        print(-1)
else:
    ans = 1
    x = 1
    while x*m_a < m:
        x *= s
        ans += 1
    print(ans)