```go
package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	scanner.Split(bufio.ScanWords)

	if !scanner.Scan() {
		return
	}
	n, _ := strconv.Atoi(scanner.Text())

	if !scanner.Scan() {
		return
	}
	m, _ := strconv.Atoi(scanner.Text())

	obsRow := make([]int, n)
	for i := range obsRow {
		obsRow[i] = -1
	}
	obsCol := make([]int, m)
	for i := range obsCol {
		obsCol[i] = -1
	}

	numObstacles := 0

	for i := 0; i < n; i++ {
		if !scanner.Scan() {
			break
		}
		line := scanner.Text()
		for j := 0; j < m && j < len(line); j++ {
			if line[j] == 'X' {
				obsRow[i] = j
				obsCol[j] = i
				numObstacles++
			}
		}
	}

	R := make([]int64, n)
	for i := 0; i < n; i++ {
		if obsRow[i] != -1 {
			R[i] = int64(m - 1)
		} else {
			R[i] = int64(m)
		}
	}

	C := make([]int64, m)
	for j := 0; j < m; j++ {
		if obsCol[j] != -1 {
			C[j] = int64(n - 1)
		} else {
			C[j] = int64(n)
		}
	}

	var sumR int64 = 0
	for i := 0; i < n; i++ {
		for j := 0; j < n; j++ {
			diff := int64(i - j)
			if diff < 0 {
				diff = -diff
			}
			sumR += R[i] * R[j] * diff
		}
	}

	var sumC int64 = 0
	for i := 0; i < m; i++ {
		for j := 0; j < m; j++ {
			diff := int64(i - j)
			if diff < 0 {
				diff = -diff
			}
			sumC += C[i] * C[j] * diff
		}
	}

	var penaltyR int64 = 0
	for i := 0; i < n; i++ {
		if obsRow[i] != -1 {
			k := int64(obsRow[i])
			L := k
			REmpty := int64(m) - 1 - k
			penaltyR += 4 * L * REmpty
		}
	}

	var penaltyC int64 = 0
	for j := 0; j < m; j++ {
		if obsCol[j] != -1 {
			k := int64(obsCol[j])
			U := k
			D := int64(n) - 1 - k
			penaltyC += 4 * U * D
		}
	}

	totalSum := sumR + sumC + penaltyR + penaltyC
	E := int64(n*m - numObstacles)

	avg := float64(totalSum) / float64(E*E)
	fmt.Printf("%.9f\n", avg)
}
```