// No.1134 Deviation Score Ⅱ
package main

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

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Buffer(make([]byte, 100000*7+2), 100000*7+2)
	sc.Scan()
	N, _ := strconv.Atoi(sc.Text())

	sc.Scan()
	ss := strings.Fields(sc.Text())
	x := make([]int, N)
	for i := 0; i < N; i++ {
		x[i], _ = strconv.Atoi(ss[i])
	}

	sc.Scan()
	M, _ := strconv.Atoi(sc.Text())

	sum := 0
	for i := 0; i < N; i++ {
		sum += x[i]
	}
	ave := float64(sum) / float64(N)

	aa := 0.0
	for i := 0; i < N; i++ {
		a := float64(x[i]) - ave
		aa += a * a
	}
	aa = math.Sqrt(aa / float64(N))
	if aa == 0 {
		fmt.Println(50)
	} else {
		bb := (float64(x[M-1]) - ave) * 10.0 / aa
		if float64(x[M-1]) >= ave {
			fmt.Println(math.Floor(50.0 + bb))
		} else {
			fmt.Println(math.Floor(50.0 - bb))
		}
	}
}