#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <queue>
#include <cstdio>
#include <ctime>
#include <assert.h>
#include <chrono>
#include <random>
#include <numeric>
#include <set>
#include <deque>
#include <stack>
using namespace std;
typedef long long int ll;
typedef unsigned long long ull;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll myRand(ll B) {
    return (ull)rng() % B;
}

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    //freopen("input.txt", "r", stdin);
    //freopen("output.txt", "w", stdout);
    int n; cin >> n;
    vector<int> a(n),b(n);
    for(int i=0;i<n;i++){
        cin >> a[i];
    }
    b=a;
    sort(b.begin(), b.end());
    if(b[0]==b[n-1]){
        printf("50\n");
        return 0;
    }
    else{
        double s=0,t=0;
        for(int i=0;i<n;i++){
            s+=a[i];
        }
        s/=n;
        for(int i=0;i<n;i++){
            t+=(s-a[i])*(s-a[i]);
        }
        t=sqrt(t/n);
        int m; cin >> m;
        m--;
        double res=50+10*(a[m]-s)/t;
        if(res<0){
            printf("%d\n",(int)(res-1e-10));
        }
        else{
            printf("%d\n",(int)(res+1e-10));
        }
    }
}