import sys import math def main(): input = sys.stdin.read().split() idx = 0 N = int(input[idx]) idx += 1 K = int(input[idx]) idx += 1 A = list(map(int, input[idx:idx+N])) if K == 1: print(0) return if K == N: print(1) return A1 = A[0] A2 = A[1] sum_terms = [ (0, 1) ] * (N + 1) sum_terms[2] = (0, 1) for j in range(3, N+1): numerator = A1 * A2 denominator = A[j-2] * A[j-1] a_prev, b_prev = sum_terms[j-1] new_num = a_prev * denominator + numerator * b_prev new_den = b_prev * denominator g = math.gcd(new_num, new_den) if g != 0: new_num //= g new_den //= g sum_terms[j] = (new_num, new_den) a, b = sum_terms[N] S_num = b + a S_den = b a_p, b_p = sum_terms[K] numerator_p = b_p + a_p denominator_p = b_p numerator = numerator_p * S_den denominator = denominator_p * S_num g = math.gcd(numerator, denominator) p = numerator // g q = denominator // g if q == 0: print(0) elif q == 1: print(p) else: print(f"{p}/{q}") if __name__ == "__main__": main()