#include <bits/stdc++.h>
using namespace std;
const long long INF = 1e18, N = 1e6, mod = 1e9 + 7;
const long double PI = acos(-1), eps = -1e9;
inline void read(long long &a) {
  char c;
  bool neg = 0;
  c = getchar();
  a = 0;
  if (c == '-') {
    neg = 1;
    c = getchar();
  }
  while (c >= '0' && c <= '9') {
    a = (a << 3) + (a << 1) + (c - '0');
    c = getchar();
  }
  if (neg) a *= -1;
}
long long a[N];
void solve() {
  long long n;
  read(n);
  long long sum = 0, cur = 0;
  for (long long i = 1; i <= n; i++) {
    long long x;
    read(x);
    if (a[x] != 0) {
      cur -= a[x];
    }
    a[x] = i;
    cur += a[x];
    sum += cur;
  }
  sum -= n;
  sum *= 2;
  sum += n;
  cout << setprecision(20) << (long double)sum / ((long double)n * n);
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
