/*
このコード、と～おれ!
Be accepted!
∧＿∧　
（｡･ω･｡)つ━☆・*。
⊂　　 ノ 　　　・゜+.
　しーＪ　　　°。+ *´¨)
 　　　　　　　　　.· ´¸.·*´¨) ¸.·*¨)
		  　　　　　　　　　　(¸.·´ (¸.·'* ☆
*/

#include <cstdio>
#include <algorithm>
#include <string>
#include <cmath>
#include <cstring>
#include <vector>
#include <numeric>
#include <iostream>
#include <random>
#include <map>
#include <unordered_map>
#include <queue>
#include <regex>
#include <functional>
#include <complex>
#include <list>
#include <cassert>
#include <iomanip>
#include <set>
#include <stack>
#include <bitset>

////多倍長整数, cpp_intで宣言
//#include <boost/multiprecision/cpp_int.hpp>
//using namespace boost::multiprecision;

//#pragma gcc target ("avx2")
//#pragma gcc optimization ("O3")
//#pragma gcc optimization ("unroll-loops")
#define rep(i, n) for(int i = 0; i < (n); ++i)
#define printynl(a) printf(a ? "yes\n" : "no\n")
#define printyn(a) printf(a ? "Yes\n" : "No\n")
#define printYN(a) printf(a ? "YES\n" : "NO\n")
#define printim(a) printf(a ? "possible\n" : "imposible\n")
#define printdb(a) printf("%.50lf\n", a) //少数出力
#define printLdb(a) printf("%.50Lf\n", a) //少数出力
#define printdbd(a) printf("%.16lf\n", a) //少数出力(桁少なめ)
#define prints(s) printf("%s\n", s.c_str()) //string出力
#define all(x) (x).begin(), (x).end()
#define deg_to_rad(deg) (((deg)/360.0L)*2.0L*PI)
#define rad_to_deg(rad) (((rad)/2.0L/PI)*360.0L)
#define Please return
#define AC 0
#define manhattan_dist(a, b, c, d) (abs(a - c) + abs(b - d)) /*(a, b) から (c, d) のマンハッタン距離 */
#define inf numeric_limits<double>::infinity();
#define linf numeric_limits<long double>::infinity()

using ll = long long;
using ull = unsigned long long;

constexpr int INF = 1073741823;
constexpr int MINF = -1073741823;
constexpr ll LINF = ll(4661686018427387903);
constexpr ll MOD = 1e9 + 7;
constexpr long double eps = 1e-6;
const long double PI = acosl(-1.0L);

using namespace std;

void scans(string& str) {
	char c;
	str = "";
	scanf("%c", &c);
	if (c == '\n')scanf("%c", &c);
	while (c != '\n' && c != -1 && c != ' ') {
		str += c;
		scanf("%c", &c);
	}
}

void scanc(char& str) {
	char c;
	scanf("%c", &c);
	if (c == -1)return;
	while (c == '\n') {
		scanf("%c", &c);
	}
	str = c;
}

double acot(double x) {
	return PI / 2 - atan(x);
}

ll LSB(ll n) { return (n & (-n)); }

template<typename T>
T chmin(T& a, const T& b) {
	if (a > b)a = b;
	return a;
}

template<typename T>
T chmax(T& a, const T& b) {
	if (a < b)a = b;
	return a;
}

/*-----------------------------------------ここからコード-----------------------------------------*/



int main() {

	int n;
	char c;
	scanf("%d%c", &n, &c);
	assert(1 <= n);
	assert(n <= 1e5);
	assert(c == '\n');
	vector<int> x(n);
	long double sum = 0;
	set<int> st;
	rep(i, n) {
		scanf("%d%c", &x[i], &c);
		st.insert(x[i]);
		assert(0 <= x[i]);
		assert(x[i] <= 100);
		assert(i == n - 1 ? c == '\n' : c == ' ');
		sum += x[i];
	}
	if (st.size() == 1) {
		puts("50");
		return 0;
	}
	sum /= n;
	int m;
	scanf("%d%c", &m, &c);
	assert(1 <= m and m <= n and c == '\n');
	--m;
	vector<long double> k(n);
	rep(i, n)k[i] = pow((long double)x[i] - sum, 2);
	long double ave = 0;
	rep(i, n)ave += k[i];
	ave /= n;
	ave = sqrt(ave);
	long double ans = (long double)x[m] - sum;
	ans *= 10;
	ans /= ave;
	ans += 50.0;
	printf("%d\n", ans > 0 ? (int)floor(ans) : (int)ceil(ans));

	Please AC;
}