
    A bus company needs to calculate the total passenger load factor across a bus route that starts from the city center. Given the total number of stops on a route, passenger density follows a predictable pattern where each stop pair moving away from the center shows a decrease of 2 units in passenger volume compared to the previous pair. The calculation should sum up the passenger densities starting from the central stop until reaching the suburbs where passenger volume becomes negligible (zero or negative). This passenger load pattern helps determine optimal bus frequency and capacity allocation throughout the route's duration. The density values begin with the maximum at the first stop (n) and decrease as (n-2), (n-4), and so on for each subsequent pair of stops.
    

    
def sum_series(n):
    """
    A bus company needs to calculate the total passenger load factor across a bus route that starts from the city center. Given the total number of stops on a route, passenger density follows a predictable pattern where each stop pair moving away from the center shows a decrease of 2 units in passenger volume compared to the previous pair. The calculation should sum up the passenger densities starting from the central stop until reaching the suburbs where passenger volume becomes negligible (zero or negative). This passenger load pattern helps determine optimal bus frequency and capacity allocation throughout the route's duration. The density values begin with the maximum at the first stop (n) and decrease as (n-2), (n-4), and so on for each subsequent pair of stops.
    

    """
def sum_series(n):
    """
    A bus company needs to calculate the total passenger load factor across a bus route that starts from the city center. Given the total number of stops on a route, passenger density follows a predictable pattern where each stop pair moving away from the center shows a decrease of 2 units in passenger volume compared to the previous pair. The calculation should sum up the passenger densities starting from the central stop until reaching the suburbs where passenger volume becomes negligible (zero or negative). This passenger load pattern helps determine optimal bus frequency and capacity allocation throughout the route's duration. The density values begin with the maximum at the first stop (n) and decrease as (n-2), (n-4), and so on for each subsequent pair of stops.
    

    """
    if n < 1:
        return 0
    else:
        return n + sum_series(n - 2)