POJ 2601

#include<iostream>
#include<iomanip>
#include<stdio.h>
using namespace std;

int main()
{
    //freopen("acm.acm","r",stdin);
    int n;
    int i;
    double * a;
    double * c;
    cin>>n;
    a = new double[n+2];
    c = new double[n];
    
    cin>>a[0]>>a[n+1];
    for(i = 0; i < n; ++ i)
    {
        cin>>c[i];
    }
    a[1] = n*a[0] + a[n+1];
    for(i = 0; i < n; ++ i)
    {
        a[1] -= 2*(n - i)*c[i];
    }
    cout<<setiosflags(ios::fixed)<<setprecision(2)<<a[1]/(n+1)<<endl;
}