杭电2086

公式A[i+1]=2A[i]-A[i-1]+2C[i]
        A[i+1]=3A[i-1]-2A[i-2]+4c[i-1]+2C[i]
        A[i+1]=4A[i-2]-3A[i-3]+6c[i-2]+4c[i-1]+2C[i]
        ...

        A[n+1]=(n+1)A[1]-nA[0]+2*(n*c1+(n-1)*c2+......cn);

#include<iostream>
#include<cstdio>
using namespace std;

int main(){
   int n;
   double a1,an1;
   double c[3000];
   while(scanf("%d",&n)!=EOF){
     scanf("%lf%lf",&a1,&an1);
     for(int i = 0;i<n;i++)
		 scanf("%lf",&c[i]);
	 double sumc = 0;
	 for(int i = 0;i<n;i++){
	    sumc+=c[i]*(n-i);
	 }
	 double res = (n*a1+an1-2*sumc)/(n+1);
	 printf("%.2lf\n",res);
   }
}