#include<bits/stdc++.h>
using namespace std;
long long c[100010];
int main(){
	long long n, m, s1, p1, s2, p2, qi = 0; cin >> n;
	for (int i = 1; i <= n; i++) cin >> c[i];
	cin >> m >> p1 >> s1 >> s2; c[p1] += s1;
	for (int i = 1; i <= n; i++) qi += (m - i) * c[i];
	cout << m + 1.0 * qi / s2 << endl;
	return 0;
}