#include "stdio.h"

double cal(int t,double a[]);

int main(){

    double a0,an1,c[],sum,k;

    int i,n,b;

    while(~scanf("%d",&n)){

        scanf("%lf%lf",&a0,&an1);

        for(i=;i<=n;i++)

            scanf("%lf",&c[i]);

        sum=(double)n/(double)(n+)*a0+1.0/(n+)*an1;

        k=(double)n*2.0;

        for(i=;i<=n;i++){

            sum-=k/(n+)*c[i];

            k-=2.0;

        }

        printf("%.2lf\n",sum);

    }

    return ;

}