import math

def main():
    import sys
    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)
        return
    
    # Calculate sum_r (from m=2 to N)
    sum_r_num = 0
    sum_r_den = 1
    for m in range(2, N+1):
        prev_index = m - 2
        curr_index = m - 1
        a_prev = A[prev_index]
        a_curr = A[curr_index]
        denom = a_prev * a_curr
        term_num = 1
        term_den = denom
        
        new_num = sum_r_num * term_den + sum_r_den * term_num
        new_den = sum_r_den * term_den
        g = math.gcd(new_num, new_den)
        sum_r_num = new_num // g
        sum_r_den = new_den // g
    
    # Calculate sum_prime (from m=2 to K)
    sum_prime_num = 0
    sum_prime_den = 1
    for m in range(2, K+1):
        prev_index = m - 2
        curr_index = m - 1
        a_prev = A[prev_index]
        a_curr = A[curr_index]
        denom = a_prev * a_curr
        term_num = 1
        term_den = denom
        
        new_num = sum_prime_num * term_den + sum_prime_den * term_num
        new_den = sum_prime_den * term_den
        g = math.gcd(new_num, new_den)
        sum_prime_num = new_num // g
        sum_prime_den = new_den // g
    
    # Compute the probability as sum_prime / sum_r
    prob_num = sum_prime_num * sum_r_den
    prob_den = sum_prime_den * sum_r_num
    
    if prob_num == 0:
        print(0)
        return
    
    g = math.gcd(prob_num, prob_den)
    prob_num //= g
    prob_den //= g
    
    print(f"{prob_num}/{prob_den}")

if __name__ == '__main__':
    main()