package main

import . "fmt"
import . "math"

func main() {
	var n,m int
	Scan(&n)
	x := make([]int64, n)
	var sum int64
	for i := range x {
		Scan(&x[i])
		sum += x[i]
	}
	Scan(&m)
	var sum2 int64
	for _, v := range x {
		sum2 += (v*int64(n)-sum)*(v*int64(n)-sum)
	}
	if sum2 == 0 {
		Println(50)
		return
	}
	z := float64((x[m-1]*int64(n)-sum)*10)*Sqrt(float64(n)/float64(sum2))
	if x[m-1]*int64(n)>=sum {
		Println(int(Floor(50+z)))
	} else {
		Println(int(Floor(50-z)))
	}
}