Calculate rolling summation or subtraction between parent and child fields

This script retrieves values from the Expense field in the issue subtasks, and then subtracts them from the budget.

RollingFieldSummationParentChild.sil script

string [] stasks = subtasks(key);
int budget = customfield_10714;

if(isNotNull(budget)) {
    for(string s in stasks) {
        if(isNotNull(%s%.customfield_10713)) {
            budget = budget - s.customfield_10713;
        }
    }
}

return budget;