#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
ll a[1000005],b[1000005],n,k,s;
ll m=-1e9;
int main(){
freopen("zdpj.in","r",stdin);
freopen("zdpj.out","w",stdout);
cin>>n;
for(ll i=1;i<=n;i++){
cin>>a[i];
b[i]=a[i]+b[i-1];
}
cin>>k;
for(ll i=n;i>=k;i--){
s=b[i]-b[i-k];
m=max(s,m);
}
printf("%.2lf",m*1.0/k);
return 0;
}