Given a sequence of positive numbers, a segment is defined to be a consecutive subsequence. For example, given the sequence { 0.1, 0.2, 0.3, 0.4 }, we have 10 segments: (0.1) (0.1, 0.2) (0.1, 0.2, 0.3) (0.1, 0.2, 0.3, 0.4) (0.2) (0.2, 0.3) (0.2, 0.3, 0.4) (0.3) (0.3, 0.4) and (0.4).

Now given a sequence, you are supposed to find the sum of all the numbers in all the segments. For the previous example, the sum of all the 10 segments is 0.1 + 0.3 + 0.6 + 1.0 + 0.2 + 0.5 + 0.9 + 0.3 + 0.7 + 0.4 = 5.0.

## Input Specification:

Each input file contains one test case. For each case, the first line gives a positive integer N, the size of the sequence which is no more than $10^5$. The next line contains N positive numbers in the sequence, each no more than 1.0, separated by a space.

## Output Specification:

For each test case, print in one line the sum of all the numbers in all the segments, accurate up to 2 decimal places.

4
0.1 0.2 0.3 0.4

5.00

## 解答

#include<iostream>
#include<vector>
using namespace std;
int main()
{
int n;
cin >> n;
double sum = 0.0, k;
for (int i = 1; i <= n; i++) {
cin >> k;
sum += k * i * (n - i + 1);
}
printf("%.2f", sum);
return 0;
}