#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = false;
//---------------------------------//

int main() {
	int N;
	cin >> N;
	vector<int> X(N);
	REP(i, N) scanf("%d", &X[i]);
	int M;
	cin >> M;
	--M;
	
	if (all_of(ALL(X), [&](int x) { return x == X[0]; })) puts("50");
	else {
		int s1 = accumulate(ALL(X), 0);
		int s2 = 0;
		REP(i, N) s2 += X[i] * X[i];
		double r = sqrt(s2 / (double)N - pow(s1 / (double)N, 2));
		double ans = 10 * (X[M] - s1 / (double)N) / r + 50;
		int asi;
		if (ans > -1e-9) {
			// 正
			if (ans < 0) asi = 0;
			else {
				asi = floor(ans);
				if (asi + 1 - ans <= 1e-9) ++asi;
			}
		}
		else {
			asi = ceil(ans);
			if (ans - (asi - 1) <= 1e-9) --asi;
		}
		cout << asi << endl;
	}
}