import sys from math import gcd 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_c_num = 0 sum_c_den = 1 sum_k_num = 0 sum_k_den = 1 current_num = 0 current_den = 0 for i in range(2, N+1): if i == 2: current_num = 1 current_den = 1 else: a_prev = A[i-3] if (i-3) >= 0 else 1 a_i = A[i-1] current_num *= a_prev current_den *= a_i g = gcd(current_num, current_den) current_num //= g current_den //= g new_num = sum_c_num * current_den + current_num * sum_c_den new_den = sum_c_den * current_den g = gcd(new_num, new_den) sum_c_num = new_num // g sum_c_den = new_den // g if i <= K: new_k_num = sum_k_num * current_den + current_num * sum_k_den new_k_den = sum_k_den * current_den g = gcd(new_k_num, new_k_den) sum_k_num = new_k_num // g sum_k_den = new_k_den // g if sum_c_num == 0: print("0") return numerator = sum_k_num * sum_c_den denominator = sum_k_den * sum_c_num g = gcd(numerator, denominator) numerator //= g denominator //= g if numerator == 0: print("0") else: print(f"{numerator}/{denominator}") if __name__ == '__main__': main()