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

int main(){
  int n, m;
  cin >> n >> m;
  ll c; cin >> c;
  vector<ll> a(n), b(m+1);
  for(int i = 0; i < n; i++) cin >> a[i];
  for(int i = 0; i < m; i++) cin >> b[i];
  b[m] = 1e9;
  sort(b.begin(), b.end());
  ll cnt = 0;
  for(int i = 0; i < n; i++){
    ll num = c/a[i];
    int l = 0; int r = m;
    while(r-l>1){
      int mid = (l+r)/2;
      if(b[mid]<=num) l = mid;
      else r = mid;
    }
    if(b[0]>num) cnt += m;
    else cnt += m - r;
  }
  double ans = 0;
  ans = cnt/(1.0*m*n);
  printf("%.10f\n",ans);
  return 0;
}