hdu 2086

#include "stdio.h"
double cal(int t,double a[]);
int main(){
    double a0,an1,c[1100],sum,k;
    int i,n,b;
    while(~scanf("%d",&n)){
        scanf("%lf%lf",&a0,&an1);
        for(i=1;i<=n;i++)
            scanf("%lf",&c[i]);
        sum=(double)n/(double)(n+1)*a0+1.0/(n+1)*an1;
        k=(double)n*2.0;
        for(i=1;i<=n;i++){
            sum-=k/(n+1)*c[i];
            k-=2.0;
        }
        printf("%.2lf\n",sum);
    }
    return 0;
}