import math

def main():
    import sys
    input = sys.stdin.read
    data = input().split()
    n = int(data[0])
    k = int(data[1])
    a = list(map(int, data[2:2+n]))
    
    if n == 1:
        print(0)
        return
    
    # Compute sum_S
    sum_num = 0
    sum_den = 1
    for i in range(n-1):
        ai = a[i]
        ai1 = a[i+1]
        term_num = 1
        term_den = ai * ai1
        
        new_num = sum_num * term_den + term_num * sum_den
        new_den = sum_den * term_den
        
        g = math.gcd(new_num, new_den)
        sum_num = new_num // g
        sum_den = new_den // g
    
    # Compute sum_Sk
    sum_k_num = 0
    sum_k_den = 1
    for i in range(k-1):
        ai = a[i]
        ai1 = a[i+1]
        term_num = 1
        term_den = ai * ai1
        
        new_num = sum_k_num * term_den + term_num * sum_k_den
        new_den = sum_k_den * term_den
        
        g = math.gcd(new_num, new_den)
        sum_k_num = new_num // g
        sum_k_den = new_den // g
    
    # Compute probability
    numerator = sum_k_num * sum_den
    denominator = sum_k_den * sum_num
    
    g = math.gcd(numerator, denominator)
    p = numerator // g
    q = denominator // g
    
    if q == 0:
        print(0)
    else:
        if q == 1:
            print(p)
        else:
            print(f"{p}/{q}")

if __name__ == '__main__':
    main()