#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n); i++)
typedef long long ll;
typedef long double ld;
typedef pair<int,int> P;

int main() {  
  int n, m;
  ll c;
  cin >> n >> m >> c;
  vector<ll> a(n), b(m);
  rep(i,n) cin >> a[i];
  rep(i,m) cin >> b[i];
  sort(b.begin(), b.end());
  ld cnt = 0;
  ld sum = (ld)n * m;
  for (int i = 0; i < n; i++) {
    ll x = c / a[i];
    cnt += m - (upper_bound(b.begin(), b.end(), x) - b.begin()); 
  }
  printf("%.12Lf\n", cnt / sum);
  return 0;
}