import sys
input = lambda : sys.stdin.readline().rstrip()

sys.setrecursionlimit(2*10**5+10)
write = lambda x: sys.stdout.write(x+"\n")
debug = lambda x: sys.stderr.write(x+"\n")

from fractions import Fraction as f
n,k = list(map(int, input().split()))
a = list(map(int, input().split()))
if n==1:
    ans = 1,1
elif n==2:
    if k==2:
        ans = 1,1
    else:
        ans = 0,1
else:
    if k==1:
        ans = 0,1
    elif k==n:
        ans = 1,1
    else:
        s = [0,1]
        t = [0,0]
        for i in range(n-2):
            a0,a1 = a[i],a[i+2]
            ns = f((a0+a1)*s[-1] - a0*s[-2], a1)
            nt = f((a0+a1)*t[-1] - a0*t[-2], a1)
            s.append(ns)
            t.append(nt)
        val = f(1-t[-1], s[-1])
        val = val * s[k-1] + t[k-1]
        ans = val.numerator, val.denominator
if ans[0]==0:
    print(0)
else:
    from math import gcd
    g = gcd(ans[0], ans[1])
    ans = [ans[0]//g, ans[1]//g]
    print("/".join(map(str, ans)))