# import pypyjit
# pypyjit.set_param('max_unroll_recursion=-1')
from collections import *
from itertools import *
from functools import *
from heapq import *
import sys,math
input = sys.stdin.readline

N,K = map(int,input().split())
A = list(map(int,input().split()))
val = [(-1,-1) for _ in range(N)]
val[1] = (1,1)
val[0] = (0,1)
def f(x):
    if val[x]!=(-1,-1):
        return val[x]
    
    p1,q1 = f(x-1)
    p0,q0 = f(x-2)
    lx,ly = A[x]+A[x-2],A[x]
    rx,ry = A[x-2],A[x]
    ax,ay = lx*p1,ly*q1
    bx,by = rx*p0,ry*q0
    s = ax*by - bx*ay
    t = ay*by
    g = math.gcd(s,t)
    val[x] = (s//g,t//g)
    return val[x]
pn,qn = f(N-1)
if N==1:
    print(1)
    exit()
if K==1:
    print(0)
    exit()
pk,qk = f(K-1)
s = pk*qn
t = pn*qk
g = math.gcd(s,t)
s //=g
t //=g
print(str(s)+'/'+str(t))