HDU 2086 A1 = ?

#include <bits/stdc++.h>
using namespace std;

double c,a0,ann;

int main() {
    int n;
    while(~scanf("%d", &n)) {
        scanf("%lf%lf", &a0, &ann);
        int sum = 0;
        for(int i = 1; i <= n; i++) {
            scanf("%lf", &c);
            sum+=c*(n + 1 - i);
        }
        printf("%.2f\n",(a0 * n + ann - 2 * sum) / (n + 1));
    }
    return 0;
}