from math import gcd, lcm

N, K = map(int, input().split())
A = list(map(int, input().split()))

if N == 1:
    print("1/1")
    exit()

if K == 1:
    print("0")
    exit()

lcm_all = lcm(*((i * j) for i in range(1, 11) for j in range(1, 11)))

dp = [0] * N
for i in range(N - 1):
    dp[i + 1] = dp[i] + lcm_all // (A[i] * A[i + 1])
denominator = dp[-1]
numerator = dp[K - 1]
g = gcd(numerator, denominator)
denominator //= g
numerator //= g
print(f"{numerator}/{denominator}")