from fractions import Fraction
n,k = map(int, input().split())
a = list(map(int, input().split()))
x1 = [Fraction(0, 1)]
x0 = [Fraction(0, 1)]
for i in range(1, n-1):
    y = Fraction(a[i-1], a[i-1] + a[i+1])
    z = Fraction(a[i+1], a[i-1] + a[i+1])
    nxd = Fraction(1) - y * x1[-1]
    x1.append(z / nxd)
    x0.append(y * x0[-1] / nxd)

p = [Fraction(0) for _i in range(n)]
p[-1] = 1

for ri in range(1, n-1):
    i = n - 1 - ri
    p[i] = x1[i] * p[i+1] + x0[i]

print(p[k-1])