import sys,random,bisect
from collections import deque,defaultdict
from heapq import heapify,heappop,heappush
from itertools import permutations
from math import log,gcd

input = lambda :sys.stdin.buffer.readline()
mi = lambda :map(int,input().split())
li = lambda :list(mi())

N,K = mi()
A = li()
n = N

if N==1:
    print("1/1")
    exit()

dp = [[(0,1),(0,1)] for i in range(n)]
dp[1][1] = (1,1)
for i in range(2,n):
    p,q = A[i-2],A[i-2]+A[i]

    a,b = dp[i-1][1]
    c,d = dp[i-2][1]
    c,d = c*p,d*q

    a,b,c,d = a*d,b*d,c*b,b*d
    p,q = A[i-2],A[i-2]+A[i]

    e,f = (a-c)*q,(q-p)*b
    g = gcd(e,f)
    dp[i][1] = (e//g,f//g)

a,b = dp[n-1][0]
c,d = dp[n-1][1]

"""
1 = a/b + c/d * dp[1]
dp[1] = -a*d/b*c
"""

e,f = (b-a)*d,b*c
g = gcd(e,f)
e //= g
f //= g

a,b = dp[K-1][0]
c,d = dp[K-1][1]

"""
p/q = a/b + c/d * e/f
    = adf/bdf + bce/bdf
    = (a*d*f+b*c*e)/(b*d*f)
"""

p,q = a*d*f+b*c*e,b*d*f
if p<0:
    p,q = -p,-q
g = gcd(p,q)
if p:
    print("{}/{}".format(p//g,q//g))
else:
    print(0)