1104 Sum of Number Segments （20 point(s)）
#include<iostream>
#include<cstdio>
using namespace std;
double res, x;
int n;
int main() {
scanf("%d", &n);
for(int i = 1; i <= n; ++i) scanf("%lf", &x), res += x * (n - i + 1) * i;
printf("%.2f\n", res);
return 0;
}