#include<iostream>
#include<cmath>
using namespace std;
int N;
int x[1<<17];
int M;
main()
{
	cin>>N;
	long s2=0,s=0;
	for(int i=0;i<N;i++)
	{
		cin>>x[i];
		s+=x[i];
		s2+=x[i]*x[i];
	}
	cin>>M;M--;
	cout<<(int)(50+10*(N*x[M]-s)/sqrt(N*s2-s*s))<<endl;
}