import math
def plus(a,b,c,d):
  x,y=(a*d+b*c),b*d
  g=math.gcd(x,y)
  if x<=0 and y<=0:
      x,y=-x,-y
  return x//g,y//g
N,K=map(int,input().split())
A=list(map(int,input().split()))
N=3*10**4
K=1
A=[1,10,10,4,9]*N
L=[0]*N
L[0]=[0,1]
L[1]=[1,1]
for i in range(1,N-1):
  a=A[i-1]
  b=A[i+1]
  n1,m1=L[i-1]
  n2,m2=L[i]
  x,y=plus((a+b)*n2,m2,-n1*a,m1)
  g=math.gcd(x,y*b)
  L[i+1]=x//g,y*b//g
a,b=L[K-1]
c,d=L[-1]
x,y=a*d,c*b
g=math.gcd(x,y)
x,y=x//g,y//g
if x==0:
  print(0)
  exit()
print(str(x)+"/"+str(y))