from fractions import Fraction as F
def main():
    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}')

if __name__ == '__main__':
    main()