package main

import (
	"fmt"
	"io"
	"os"
)

func main() {
	buf, _ := io.ReadAll(os.Stdin)
	var pos int
	nextInt := func() int {
		for pos < len(buf) && (buf[pos] < '0' || buf[pos] > '9') {
			pos++
		}
		if pos >= len(buf) {
			return 0
		}
		res := 0
		for pos < len(buf) && buf[pos] >= '0' && buf[pos] <= '9' {
			res = res*10 + int(buf[pos]-'0')
			pos++
		}
		return res
	}

	n := nextInt()
	if n == 0 {
		return
	}

	last := make([]int, 1000005)
	seen := make([]bool, 1000005)

	var sumExcluded int64 = 0
	var distinctCount int64 = 0

	for i := 1; i <= n; i++ {
		val := nextInt()
		if !seen[val] {
			seen[val] = true
			distinctCount++
		}
		L := int64(i - last[val] - 1)
		sumExcluded += L * L
		last[val] = i
	}

	for val, isSeen := range seen {
		if isSeen {
			L := int64(n - last[val])
			sumExcluded += L * L
		}
	}

	expected := float64(distinctCount) - float64(sumExcluded)/float64(int64(n)*int64(n))
	fmt.Printf("%.6f\n", expected)
}