from collections import Counter
n,m,c = map(int,input().split())
a = list(map(int,input().split()))
b = list(map(int,input().split()))

a = sorted(a)[::-1]
b_counter = Counter(b)
b_key = sorted(list(b_counter.keys()))[::-1]

ans = 0
for i in range(n):
    elem = c // a[i]
    if(b_key[-1] > elem):
        ans += m
    else:
        for j in range(len(b_key)):
            if(b_key[j] > elem):
                ans += b_counter[b_key[j]]
            else:
                break
print(ans / (n * m))