We can use a variable $sum$ to record the sum of negative numbers under each positive number, and another variable $total$ to record the sum of all negative numbers under all positive numbers.
The specific implementation method is as follows:
1. Initially, $sum=0$, $total=0$.
2. Traverse the column number from left to right. For each $a_i$ number:
A. If $a_i$ is a positive number, add $sum$ to $total$ and set $sum$ to 0.
B. If $a_i$ is negative, add $a_i$ to $sum$.
3. After traversing the whole series, if the last number is positive, add $sum$ to $total$.
In this way, the value of $total$ is the sum of all negative numbers under positive numbers.
The mathematical formula is as follows:
$ $ total = \sum_{i= 1}^{n}[a _ I & gt； 0](\sum_{j=i+ 1}^{n}[a_j<； 0]a_j)$$
Where $ [a _ i >; 0]$ is an index function with a value of 1 or 0, which means that when $ a _ i >; 0$ is 1, otherwise it is 0; $[a _ j & lt； 0]$ is also a indicator function, which means that when $ a _ j