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

int main()
{
    double a0, an1, c;
    double a1;
    int n;
    while (cin>>n)
    {
        cin >> a0 >> an1;
        a1 = an1 + a0 * n;
        for (int i = 0; i < n; i++)
        {
            cin >> c;
            a1 -= 2 * c*(n - i);
        }
        a1 /= (n + 1);
        printf("%.2lf\n", a1);
    }

    return 0;
}