#include <bits/stdc++.h>
using namespace std;
const long long INF = 0x3f3f3f3f3f3f3f3f;
const long long MAXN = 1e6 + 7;
const long long MAXM = 2e6 + 7;
const long long MOD = 1e9 + 7;
const double eps = 1e-6;
const double pi = acos(-1.0);
long long a[MAXN];
int pre[MAXN];
int main() {
  long long n;
  while (~scanf("%lld", &n)) {
    memset(pre, 0, sizeof(pre));
    for (int i = 1; i <= n; i++) scanf("%lld", &a[i]);
    long long cnt = 0;
    for (int i = 1; i <= n; i++) {
      long long x = i - pre[a[i]], y = n - i + 1;
      cnt += x * y;
      pre[a[i]] = i;
    }
    cnt *= 2, cnt -= n;
    printf("%.7f\n", 1.0 * cnt / (n * n));
  }
  return 0;
}
