/*
惭愧啊，公式推导了老半天
数学不好的伤不起啊。。。 
*/
#define LOCAL
#include<iostream>
#include<iomanip>
using namespace std;
int main()
{
#ifdef LOCAL
       freopen("input.txt","r",stdin);
       freopen("output.txt","w",stdout);
#endif
    
    int i,n;double x,y,c,sum;
    while(cin>>n)
    {
           sum=0;
           cin>>x>>y;
           for(i=1;i<=n;i++)
           {cin>>c;sum+=2*(n+1-i)*c;}             
           sum=(n*x+y-sum)/(n+1);
           cout<<setprecision(2)<<setiosflags(ios::fixed)<<sum<<endl;
    }
    return 0;
}