#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define FOR(i,b,e) for (int i=(int)(b); i<(int)(e); i++)
#define ALL(x) (x).begin(), (x).end()

const double PI = acos(-1);

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int n, m, c;
  cin >> n >> m >> c;
  vector<long long> a(n), b(m);
  REP (i, n) cin >> a[i];
  REP (i, m) cin >> b[i];

  sort(b.begin(), b.end());
  
  long long cnt = 0;
  for (int i = 0; i < n; i++) {
    long long x = c / a[i];
    cnt += b.end() - upper_bound(b.begin(), b.end(), x);
  }

  double ret = 1.0 * cnt / n / m;
  cout << fixed << setprecision(9) << ret << endl;
    
  return 0;
}