import sys
import math

def main():
    N, K = map(int, sys.stdin.readline().split())
    A = list(map(int, sys.stdin.readline().split()))
    if K == 1:
        print("0")
        return
    if K == N:
        print("1/1")
        return
    
    # Compute S_C: sum from j=2 to N of 1/(A_{j-1} * A_j)
    sum_c_num = 0
    sum_c_den = 1
    for j in range(2, N+1):
        a_prev = A[j-2]
        a_curr = A[j-1]
        den = a_prev * a_curr
        add_num = 1
        add_den = den
        new_num = sum_c_num * add_den + sum_c_den * add_num
        new_den = sum_c_den * add_den
        g = math.gcd(new_num, new_den)
        sum_c_num = new_num // g
        sum_c_den = new_den // g
    
    # Compute S_K: sum from j=2 to K of 1/(A_{j-1} * A_j)
    sum_k_num = 0
    sum_k_den = 1
    for j in range(2, K+1):
        a_prev = A[j-2]
        a_curr = A[j-1]
        den = a_prev * a_curr
        add_num = 1
        add_den = den
        new_num = sum_k_num * add_den + sum_k_den * add_num
        new_den = sum_k_den * add_den
        g = math.gcd(new_num, new_den)
        sum_k_num = new_num // g
        sum_k_den = new_den // g
    
    # Calculate the probability as (sum_k * sum_c_den) / (sum_k_den * sum_c_num)
    numerator = sum_k_num * sum_c_den
    denominator = sum_k_den * sum_c_num
    if denominator == 0 or numerator == 0:
        print("0")
        return
    g = math.gcd(numerator, denominator)
    p = numerator // g
    q = denominator // g
    print(f"{p}/{q}")

if __name__ == "__main__":
    main()