import sys
import math
from fractions import Fraction

def main():
    sys.setrecursionlimit(1 << 25)
    N, K = map(int, sys.stdin.readline().split())
    A = list(map(int, sys.stdin.readline().split()))
    A = [0] + A  # make it 1-based

    if K == 1:
        print("0")
        return
    if K == N:
        print("1")
        return

    # Precompute prefix_num and prefix_den
    prefix_num = [1] * (N + 1)
    for i in range(1, N + 1):
        prefix_num[i] = prefix_num[i - 1] * A[i]

    prefix_den = [1] * (N + 1)
    for j in range(3, N + 1):
        prefix_den[j] = prefix_den[j - 1] * A[j]

    # Compute sum_T (sum of Tj for j=3 to N)
    sum_T = Fraction(0, 1)
    for j in range(3, N + 1):
        num = prefix_num[j - 2]
        den = prefix_den[j]
        Tj = Fraction(num, den)
        sum_T += Tj

    S = Fraction(1, 1) + sum_T
    D2 = Fraction(1, S)

    # Compute sum_Tk (sum of Tj from j=2 to K)
    sum_Tk = Fraction(0, 1)
    for j in range(2, K + 1):
        if j == 2:
            Tj = Fraction(1, 1)
        else:
            num = prefix_num[j - 2]
            den = prefix_den[j]
            Tj = Fraction(num, den)
        sum_Tk += Tj

    P_K = D2 * sum_Tk
    p = P_K.numerator
    q = P_K.denominator

    if q == 1:
        print(p)
    else:
        print(f"{p}/{q}")

if __name__ == '__main__':
    main()