#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;   }   