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

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

	long double ave = 0;
	for( int i = 0; i < N; i++ ) {
		ave += x[i];
	}
	ave /= N;
	long double dxave = 0;
	for( int i = 0; i < N; i++ ) {
		int dx = x[i] - ave;
		dxave += dx * dx;
	}
	dxave /= N;
	long double sq = sqrt( dxave );
	int ans = 50;
	if( sq ) {
		int xm = x[M - 1];
		long double dx = xm - ave;
		long double dx2 = dx * 10 / sq;
		ans += dx2;
	}

	cout << ans << endl;
}