mod = 1000000007
eps = 10**-9


def main():
    import sys
    from math import gcd
    input = sys.stdin.readline

    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()

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

    upper = [0] * N
    lower = [1] * N
    for i in range(1, N-1):
        a = A[i-1]
        b = A[i+1]
        upper[i] = lower[i-1] * b
        lower[i] = lower[i-1] * (a+b) - a * upper[i-1]
        g = gcd(upper[i], lower[i])
        upper[i] //= g
        lower[i] //= g

    u = l = 1
    for i in range(K-1, N-1):
        u *= upper[i]
        l *= lower[i]
        g = gcd(u, l)
        u //= g
        l //= g
    if u == 0:
        print(0)
    else:
        print(str(u) + "/" + str(l))


if __name__ == '__main__':
    main()