杭电2086 A=？

# include <iostream># include <cstdio>using namespace std;int main(){

int n,i,j;double  a[2],b[50000],k;
while(cin>>n){
cin>>a[0]>>a[1];for(i=0;i<n;i++){
cin>>b[i];} double sum = 0;sum = 1.0*n/(n+1)*a[0] + 1.0/(n+1)*a[1];
k = 1.0*n*2;for(j=0;j<n;j++){
sum -= k/(n+1)*b[j];k-=2.0;}
printf("%.2lf\n",sum);}
return 0;}