from fractions import Fraction as F
n,k,*a = map(int,open(0).read().split())
a = [0]+[F(a[i-1],a[i-1]+a[i+1]) for i in range(1,n-1)]
r = [F(n==1),F(1)]
for i in range(1,n-1):
    r.append((r[-1]-r[-2]*a[i])/(1-a[i]))
A = r[k-1]/r[-1]
print(f"{A.numerator}/{A.denominator}" if A else 0)