#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
const int INF = 0x3f3f3f3f;
const ll LINF = 0x3f3f3f3f3f3f3f3fLL;
const double EPS = 1e-8;
const int MOD = 1000000007;
// const int MOD = 998244353;
const int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
const int dy8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dx8[] = {0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  int n; cin >> n;
  vector<int> x(n); REP(i, n) cin >> x[i];
  double ave = 1.0 * accumulate(ALL(x), 0) / n, den = 0;
  REP(i, n) den += (ave - x[i]) * (ave - x[i]);
  den = sqrt(den / n);
  if (den < EPS) {
    cout << 50 << '\n';
    return 0;
  }
  int m; cin >> m; --m;
  double ans = 50 + 10 * (x[m] - ave) / den;
  cout << static_cast<int>(ans < 0 ? ceil(ans) : floor(ans)) << '\n';
  return 0;
}