from fractions import Fraction
n, k=map(int, input().split())
a=list(map(int, input().split()))
if k==n:
    print("1/1")
    exit()
elif k==1:
    print(0)
    exit()
k-=1
q=[Fraction(0)]*n
q[1]=Fraction(1)
for i in range(2, n):
    q[i]=((a[i-2]+a[i])*q[i-1]-a[i-2]*q[i-2])/Fraction(a[i])
print(q[k]/q[n-1])