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

int main(){
    int N, M;
    cin >> N;
    vector<ll> x(N);
    ll S1 = 0, S2 = 0;
    for(int i = 0; i < N; i++){
        cin >> x[i];
        S1 += x[i];
        S2 += x[i] * x[i];
    }
    cin >> M;
    ll V = N * S2 - S1 * S1, a = N * x[M - 1] - S1;
    if(V == 0){
        cout << 50 << endl;
        return 0;
    }
    ll A2 = 100 * a * a, q = A2 / V, f = sqrtl(q);
    while(f * f > q) f--;
    while((f + 1) * (f + 1) <= q) f++;
    bool frac = f * f * V != A2;
    if(a >= 0) cout << 50 + f << endl;
    else if(frac && f < 50) cout << 49 - f << endl;
    else cout << 50 - f << endl;
}