#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 9;
int a[N], pos[N], pre[N];
inline int read() {
  int s = 1, a = 0;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') s = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    a = a * 10 + ch - '0';
    ch = getchar();
  }
  return s * a;
}
int main() {
  int n = read();
  for (int i = 1; i <= n; i++) a[i] = read(), pre[i] = pos[a[i]], pos[a[i]] = i;
  long long res = 0;
  for (int i = 1; i <= n; i++) res += 1ll * (i - pre[i]) * (n - i + 1);
  res = res * 2 - n;
  return 0 * printf("%.6lf\n", 1.0 * res / (1ll * n * n));
}
