#include <bits/stdc++.h>
using namespace std;

using ll = long long;

int main()
{
	ll N, M;
	cin >> N;
	vector<ll> x( N );
	for( int i = 0; i < N; i++ ) cin >> x[i];
	cin >> M;
    M--;

	ll s = 0;
	for( int i = 0; i < N; i++ ) s += x[i];
    if(x[M] * N == s) {
        cout << 50 << endl;
        return 0;
    }
    ll a = 0;
    for(int i = 0; i < N; i++) {
        ll dx = x[i] * N - s;
        a += dx * dx;
    }
    ll ans = 50 + (x[M] * N - s) / sqrt(a/ N) * 10;
    cout << ans << endl;
}