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