#include <bits/stdc++.h>
using namespace std;
inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T>
inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}
template <class T>
inline T sqr(T x) {
  return x * x;
}
const double EPS = 1e-10;
const double PI = acos(-1.0);
double war = pow(10, 9) + 7;
long long facctorialMethod(int k) {
  long long sum = 1;
  for (int i = 1; i <= k; ++i) {
    sum *= i;
  }
  return sum;
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n;
  cin >> n;
  vector<long long> x(n);
  for (int i = 0; i < n; i++) cin >> x[i];
  long long sum = 0;
  for (int i = 0; i < n - 1; i++) sum += x[i + 1];
  double kitai = (double)sum / (n - 1);
  double res = kitai * facctorialMethod(n - 1);
  cout << res << endl;
  return 0;
}
