import sys
import math

def gcd(a, b):
    while b:
        a, b = b, a % b
    return a

def simplify(num, den):
    if den == 0:
        return (0, 1)
    g = gcd(abs(num), abs(den))
    return (num // g, den // g)

def multiply(f1, f2):
    num = f1[0] * f2[0]
    den = f1[1] * f2[1]
    return simplify(num, den)

def add(f1, f2):
    num = f1[0] * f2[1] + f2[0] * f1[1]
    den = f1[1] * f2[1]
    return simplify(num, den)

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
    
    prefix = []
    prefix.append((1, 1))
    
    max_j = N - 2
    for j in range(1, max_j + 1):
        if j + 1 >= len(A):
            break
        a = A[j-1]
        b = A[j+1]
        current = multiply(prefix[j-1], (a, b))
        prefix.append(current)
    
    sum_prefix = (0, 1)
    for j in range(len(prefix)):
        sum_prefix = add(sum_prefix, prefix[j])
    
    max_j_k = K - 2
    sum_k = (0, 1)
    for j in range(max_j_k + 1):
        if j >= len(prefix):
            break
        sum_k = add(sum_k, prefix[j])
    
    if sum_prefix[1] == 0:
        print(0)
        return
    
    numerator = sum_k[0] * sum_prefix[1]
    denominator = sum_k[1] * sum_prefix[0]
    
    g = gcd(numerator, denominator)
    numerator //= g
    denominator //= g
    
    if denominator == 0:
        print(0)
    elif numerator == 0:
        print(0)
    else:
        if denominator == 1:
            print(numerator)
        else:
            print(f"{numerator}/{denominator}")

if __name__ == '__main__':
    main()