#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#include <time.h>
#include <stack>
#include <array>
#define popcount __builtin_popcount
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;

int main()
{
	int n; cin>>n;
	int x[100010];
	for(int i=0; i<n; i++) cin>>x[i];
	bool same=1;
	for(int i=0; i<n-1; i++) if(x[i]!=x[i+1]) same=0;
	if(same){
	    cout<<50<<endl;
	    return 0;
	}
	double s=0, t=0;
	for(int i=0; i<n; i++) s+=x[i];
	s/=n;
	for(int i=0; i<n; i++){
		t+=(s-x[i])*(s-x[i]);
	}
	t=sqrt(t/n);
	int m; cin>>m; m--;
	double ans=50+10*(x[m]-s)/t;
	if(ans<0) cout<<(int)(ans-1e-10)<<endl;
	else cout<<(int)(ans+1e-10)<<endl;
	return 0;
}