POJ 2601

 1 #include<iostream>
 2 #include<iomanip>
 3 #include<stdio.h>
 4 using namespace std;
 5 
 6 int main()
 7 {
 8     //freopen("acm.acm","r",stdin);
 9     int n;
10     int i;
11     double * a;
12     double * c;
13     cin>>n;
14     a = new double[n+2];
15     c = new double[n];
16     
17     cin>>a[0]>>a[n+1];
18     for(i = 0; i < n; ++ i)
19     {
20         cin>>c[i];
21     }
22     a[1] = n*a[0] + a[n+1];
23     for(i = 0; i < n; ++ i)
24     {
25         a[1] -= 2*(n - i)*c[i];
26     }
27     cout<<setiosflags(ios::fixed)<<setprecision(2)<<a[1]/(n+1)<<endl;
28 }