import sys

# sys.setrecursionlimit(1000005)
# sys.set_int_max_str_digits(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
# inf = -1-(-1 << 31)
inf = -1-(-1 << 62)

# md = 10**9+7
md = 998244353

from fractions import Fraction

n,k=LI()
aa=LI()

bb=[Fraction() for _ in range(n)]
cc=[Fraction() for _ in range(n)]
bb[1]+=1

for i in range(2,n):
    f=Fraction(-aa[i-2],aa[i])
    g=Fraction(aa[i-2]+aa[i],aa[i])
    bb[i]=f*bb[i-2]+g*bb[i-1]
    cc[i]=f*cc[i-2]+g*cc[i-1]

x=(1-cc[-1])/bb[-1]
k-=1
ans=bb[k]*x+cc[k]
if ans==0:
    print(0)
else:
    print(ans.numerator,ans.denominator,sep="/")