PAT 1104

#include<iostream>

using namespace std;

int main(){
    int num;
    cin>>num;
    double arr[100001];
    for(int i=1;i<=num;i++){
        cin>>arr[i];
        //cout<< arr[i]<<‘ ‘;
    }

    double sum=0;

    for(int k=1;k<=num;k++){
        sum += arr[k]*k*(num-k+1);
        
    }

    printf("%.2f",sum);
}