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