import sys
from fractions import Fraction

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("1/1")
        return
    
    # We compute C[j] for j from 2 to N-1 (inclusive)
    C = {}
    for j in range(2, N):
        # For the j-th position in the problem (1-based)
        # A_{j-1} is A[j-2] (0-based), A_{j+1} is A[j] (0-based)
        denominator = A[j-2] + A[j]
        a_numerator = A[j-2]
        b_numerator = A[j]
        a = Fraction(a_numerator, denominator)
        b = Fraction(b_numerator, denominator)
        
        if j == 2:
            C[j] = b
        else:
            prev_C = C[j-1]
            denominator_C = 1 - a * prev_C
            C[j] = b / denominator_C
    
    # Calculate the product from K to N-1
    result = Fraction(1, 1)
    for j in range(K, N):
        result *= C[j]
    
    if result.numerator == 0:
        print("0")
    else:
        print(f"{result.numerator}/{result.denominator}")

if __name__ == "__main__":
    main()