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")
        return
    
    sum_num = 1  # For i=2 term (1/1)
    sum_den = 1
    prefix_num = 1 if K >= 2 else 0
    prefix_den = 1 if K >= 2 else 1
    curr_num = 1
    curr_den = 1
    
    for i in range(3, N + 1):
        a_prev = A[i-3] if i-3 >= 0 else 0  # A_{i-2}
        a_next = A[i-1]  # A_i
        
        curr_num *= a_prev
        curr_den *= a_next
        
        # Update sum_terms
        new_sum_num = sum_num * curr_den + curr_num * sum_den
        new_sum_den = sum_den * curr_den
        g = math.gcd(new_sum_num, new_sum_den)
        sum_num = new_sum_num // g
        sum_den = new_sum_den // g
        
        # Update prefix_sum if current i <= K
        if i <= K:
            new_prefix_num = prefix_num * curr_den + curr_num * prefix_den
            new_prefix_den = prefix_den * curr_den
            g_p = math.gcd(new_prefix_num, new_prefix_den)
            prefix_num = new_prefix_num // g_p
            prefix_den = new_prefix_den // g_p
    
    # Calculate the resulting probability
    numerator = prefix_num * sum_den
    denominator = prefix_den * sum_num
    if denominator == 0:
        print("0")
        return
    g_total = math.gcd(numerator, denominator)
    numerator //= g_total
    denominator //= g_total
    
    if numerator == 0:
        print("0")
    else:
        print(f"{numerator}/{denominator}")

if __name__ == "__main__":
    main()