A soldier wants to buy *w* bananas in the shop. He has to pay *k* dollars for the first banana, 2*k* dollars for the second one and so on (in other words, he has to pay *i*·*k* dollars for the *i*-th banana). 

He has *n* dollars. How many dollars does he have to borrow from his friend soldier to buy *w* bananas?
The first line contains three positive integers *k*, *n*, *w* (1  ≤  *k*, *w*  ≤  1000, 0 ≤ *n* ≤ 10<sup class="upper-index">9</sup>), the cost of the first banana, initial number of dollars the soldier has and number of bananas he wants.
def soldierandbananas():
    first_banana = i = 3 
    dollars = 17
    bananas_cop = 4
    array = [None]*bananas_cop
    y = 0

    for x in range(0,bananas_cop):
        print(f"Costo banana n°{x+1}: {i}")
        array[y] = i 
        i += first_banana
        y += 1

    somma = sum(array)
    print(f"Somma: {somma}")
    print(f"Da chiedere in prestito: {somma - dollars}")