import math
n,k = map(int,input().split())
A = list(map(int,input().split()))
if n == 1 or k == n:
    print("1/1")
elif k == 1:
    print(0)
else:
    mod = 6350400
    a = 0
    for i in range(k-1):
        a += mod//(A[i]*A[i+1])
    b = 0
    for i in range(n-1):
        b += mod//(A[i]*A[i+1])
    g = math.gcd(a,b)
    a //= g
    b //= g
    print("{}/{}".format(a,b))