import sys
from fractions import Fraction

def main():
    N, K = map(int, sys.stdin.readline().split())
    A = list(map(int, sys.stdin.readline().split()))
    A = [0] + A  # Convert to 1-based indexing

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

    sum_total = Fraction(0, 1)
    sum_k = Fraction(0, 1)

    for j in range(2, N + 1):
        a_prev = A[j-1]
        a_curr = A[j]
        denom = a_prev * a_curr
        term = Fraction(1, denom)
        sum_total += term
        if j <= K:
            sum_k += term

    if sum_total == 0:
        print(0)
        return

    prob = sum_k / sum_total
    if prob.denominator == 1:
        print(prob.numerator)
    else:
        print(f"{prob.numerator}/{prob.denominator}")

if __name__ == "__main__":
    main()