from fractions import Fraction as F
N,K=map(int, input().split())
*A,=map(F, input().split())
if K == N:
    print('1/1')
elif K == 1:
    print('0')
else:
    D=[(F(0),F(0))]
    for i in range(1,N-1):
        b = A[i-1]/(A[i-1]+A[i+1])
        a = A[i+1]/(A[i-1]+A[i+1])
        bb, aa = D[-1]
        de = F(1) - b*aa
        nb = b * bb / de
        na = a / de
        D.append((nb, na))
    X = [F(0)] * N
    X[N-1] = F(1)
    for i in range(N-2, 0, -1):
        b, a = D[i]
        X[i] = b + a * X[i+1]
    print(f'{X[K-1].numerator}/{X[K-1].denominator}')