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 K == 1:
        print(0)
        return
    
    # Compute sum_S
    S_num = 0
    S_den = 1
    for j in range(2, N+1):
        a_prev = A[j-2]
        a_curr = A[j-1]
        add_den = a_prev * a_curr
        add_num = 1
        new_num = S_num * add_den + S_den * add_num
        new_den = S_den * add_den
        g = math.gcd(new_num, new_den)
        S_num = new_num // g
        S_den = new_den // g
    
    # Compute sum_K
    K_num = 0
    K_den = 1
    for j in range(2, K+1):
        a_prev = A[j-2]
        a_curr = A[j-1]
        add_den = a_prev * a_curr
        add_num = 1
        new_num = K_num * add_den + K_den * add_num
        new_den = K_den * add_den
        g = math.gcd(new_num, new_den)
        K_num = new_num // g
        K_den = new_den // g
    
    if S_num == 0:
        print(0)
        return
    
    numerator = K_num * S_den
    denominator = K_den * S_num
    if denominator == 0:
        print(0)
        return
    
    g = math.gcd(numerator, denominator)
    p = numerator // g
    q = denominator // g
    
    if q == 1:
        print(p)
    else:
        print(f"{p}/{q}")

if __name__ == "__main__":
    main()