/* -*- coding: utf-8 -*-
 *
 * 1134.cc:  No.1134 Deviation Score Ⅱ - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 100000;

/* typedef */

/* global variables */

int xs[MAX_N];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);

  int xsum = 0;
  for (int i = 0; i < n; i++) {
    scanf("%d", xs + i);
    xsum += xs[i];
  }

  int m;
  scanf("%d", &m);
  m--;

  double ave = (double)xsum / n;
  double d2sum = 0.0;
  for (int i = 0; i < n; i++) {
    double d = xs[i] - ave;
    d2sum += d * d;
  }

  double delta = sqrt(d2sum / n);

  if (delta == 0.0) puts("50");
  else {
    int d = 50.0 + (xs[m] - ave) * 10 / delta;
    printf("%d\n", d);
  }
  return 0;
}