package main

import (
	"io"
	"os"
	"strconv"
)

func main() {
	data, _ := io.ReadAll(os.Stdin)
	idx := 0
	nextInt := func() int64 {
		for idx < len(data) && (data[idx] < '0' || data[idx] > '9') {
			idx++
		}
		var v int64
		for idx < len(data) && data[idx] >= '0' && data[idx] <= '9' {
			v = v*10 + int64(data[idx]-'0')
			idx++
		}
		return v
	}

	n := int(nextInt())
	r := int(nextInt())
	m := 1 << n

	a := make([]int64, m)
	var sum int64
	for i := 0; i < m; i++ {
		a[i] = nextInt()
		sum += a[i]
	}

	den := float64(m)
	out := make([]byte, 0, (r+1)*16)

	out = strconv.AppendFloat(out, float64(sum)/den, 'f', 10, 64)
	out = append(out, '\n')

	for i := 0; i < r; i++ {
		z := int(nextInt())
		g := nextInt()
		sum += g - a[z]
		a[z] = g
		out = strconv.AppendFloat(out, float64(sum)/den, 'f', 10, 64)
		out = append(out, '\n')
	}

	os.Stdout.Write(out)
}