#include<bits/stdc++.h>
using namespace std;
long long n,v,k,a[110],maxn=INT_MIN,b[1100][1100];
double ans;
void dfs(long long lc,long long v,long long s)
{
	if(s<=b[lc][v]) return;
	b[lc][v]=s;
	if(lc==1)maxn=max(maxn,s);
	for(int i=1;i>=-1;i--)
	  if(v+i>=a[lc]&&v+i-a[lc]<=k)
	    dfs(lc-1,v+i,s+v+i);
}	
int main()
{
//	freopen(".in","r",stdin);
//	freopen(".out","w",stdout);
	scanf("%lld%lld%lld",&n,&v,&k);
	for(int i=2;i<=n;i++)
	  scanf("%lld",&a[i]);
	dfs(n,v,v);
	if(maxn==INT_MIN)cout<<"YI DING YAO JIAN CHI ZUO URAL";
	else
	{
		ans=maxn*1.0/(n-1);
		printf("%.2lf",ans);
	} 
    return 0;
}