import math
n, k = map(int,input().split())
a = list(map(int,input().split()))
l = 6350400
if(n == 1):
  print("1/1")
  exit()
if(k == 1):
  print("0")
  exit()
p = q = 0
for i in range(k - 1):
  p += l // (a[i] * a[i + 1])
for i in range(n - 1):
  q += l // (a[i] * a[i + 1])
l = math.gcd(p, q)
p //= l
q //= l
assert (p < 10**9)
assert (q < 10**9)
print(str(p) + '/' + str(q))