#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
#include <time.h> 
using namespace atcoder;
using ll = long long;
using vll = vector<ll>;
using vvll = vector<vll>;
using vvvll = vector<vvll>;
using vb = vector<bool>;
using vvb = vector<vb>;
using vvvb = vector<vvb>;
#define all(A) A.begin(),A.end()
#define rep(i, n) for (ll i = 0; i < (ll) (n); i++)
template<class T>
bool chmax(T& p, T q, bool C = 1) {
    if (C == 0 && p == q) {
        return 1;
    }
    if (p < q) {
        p = q;
        return 1;
    }
    else {
        return 0;
    }
}
template<class T>
bool chmin(T& p, T q, bool C = 1) {
    if (C == 0 && p == q) {
        return 1;
    }
    if (p > q) {
        p = q;
        return 1;
    }
    else {
        return 0;
    }
}
ll modPow(long long a, long long n, long long p) {
    if (n == 0) return 1; // 0乗にも対応する場合
    if (n == 1) return a % p;
    if (n % 2 == 1) return (a * modPow(a, n - 1, p)) % p;
    long long t = modPow(a, n / 2, p);
    return (t * t) % p;
}
ll gcd(ll(a), ll(b)) {
    if (a == 0)return b;
    if (b == 0)return a;
    ll c = a;
    while (a % b != 0) {
        c = a % b;
        a = b;
        b = c;
    }
    return b;
}
ll sqrtz(ll N) {
    ll L = 0;
    ll R = sqrt(N) + 10000;
    while (abs(R - L) > 1) {
        ll mid = (R + L) / 2;
        if (mid * mid <= N)L = mid;
        else R = mid;
    }
    return L;
}


using mint = modint998244353;
using vm = vector<mint>;
using vvm = vector<vm>;
using vvvm = vector<vvm>;


vector<mint> fact, factinv, inv;
const ll mod = 998244353;
void prenCkModp(ll n) {
    fact.resize(n + 5);
    factinv.resize(n + 5);
    inv.resize(n + 5);
    fact[0] = fact[1] = 1;
    factinv[0] = factinv[1] = 1;
    inv[1] = 1;
    for (ll i = 2; i < n + 5; i++) {
        fact[i] = (fact[i - 1] * i);
        inv[i] = (mod - ((inv[mod % i] * (mod / i))));
        factinv[i] = (factinv[i - 1] * inv[i]);
    }
}
mint nCk(ll n, ll k) {
    if (n < k || k < 0) return 0;
    return (fact[n] * ((factinv[k] * factinv[n - k])));
}


bool DEB = 1;

int main() {

    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    ll N;
    cin>>N;
    vector<double> A(N);
    rep(i,N)cin>>A[i];
    ll M;
    cin>>M;
    M--;
    double S=0.0;
    rep(i,N)S+=A[i];
    S/=double(N);
    double T=0.0;
    rep(i,N)T+=(S-A[i])*(S-A[i]);
    T/=double(N);
    T=sqrt(T);
    //cout<<T<<" "<<S<<endl;
    if(abs(T)<1e-12){
        cout<<50<<endl;
        return 0;
    }
    //cout<<"!"<<endl;
    double res=(A[M]-S)*10.0/T;
    //cout<<res<<" "<<A[M]<<endl;
    res=50.0+res;
    cout<<ll(res)<<endl;
}