from fractions import Fraction
N, K = map(int, input().split())
A = tuple(map(int, input().split()))
def solve():
    if K == N:
        print('1/1')
        return
    elif K == 1:
        print('0')
        return
    P = [Fraction(0) if i == 0 else Fraction(A[i + 1], A[i - 1] + A[i + 1]) for i in range(N - 1)]
    p2 = Fraction(1)
    p1 = Fraction(1) / P[1]
    for i in range(2, N - 1):
        p2, p1 = p1, (p1 - (1 - P[i]) * p2) / P[i]
    ans = [0] * N
    ans[0] = 0
    ans[1] = 1 / p1
    for i in range(2, N - 1):
        ans[i] = (ans[i - 1] - (1 - P[i]) * ans[i - 2]) / P[i]
    print(ans[K - 1])

solve()