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+];
     c = new double[n];

     cin>>a[]>>a[n+];
     for(i = ; i < n; ++ i)
     {
         cin>>c[i];
     }
     a[] = n*a[] + a[n+];
     for(i = ; i < n; ++ i)
     {
         a[] -= *(n - i)*c[i];
     }
     cout<<setiosflags(ios::fixed)<<setprecision()<<a[]/(n+)<<endl;
 }