#pragma GCC optimize(3,"Ofast","inline")
#include <bits/stdc++.h>
using namespace std;
inline int read()
{
	char ch(getchar());
	int x(0),m(1);
	while(!isdigit(ch)){
		if (ch=='-') m=-1;
		ch=getchar();
	}
	while(isdigit(ch)){
		x=x*10+ch-'0';
		ch=getchar();
	}
	return x*m;
}
inline void write(int x)
{
	if (x<0) putchar('-'),x=-x;
	if (x>=10) write(x/10);
	putchar(x%10+'0');
}
long long n,v,k,a[114],mx=-1234567890,b[114][114];
inline void dfs(long long t,long long s,long long sum){
	if(sum<=b[t][s]) return ;
	b[t][s]=sum;
	if(t==1){
		mx=max(mx,sum);
		return ;
	}
	for(int i=1;i>-2;i--){
		if(s+i>=a[t]&&s+i-a[t]<=k) dfs(t-1,s+i,sum+s+i);
	}
}
signed main ()
{
	n=read(),v=read(),k=read();
	for(int i=2;i<=n;i++) a[i]=read();
	dfs(n-1,v,v);
	if(mx==-1234567890){
		puts("YI DING YAO JIAN CHI ZUO URAL");
		return 0;
	}
	double ans=mx*1.0/(n-1);
	printf("%.2lf",ans);
	return 0;
}