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.
import smtplib
from smtplib import SMTPException
sender = '[email protected]'
receivers = ['[email protected]']

message = """From: From Person <[email protected]>
To: To Person <[email protected]>
Subject: SMTP e-mail test

This is a test e-mail message.
"""
gmail_user="[email protected]"
gmail_pwd="fgwoizoyjahnaxth"
# SMTP_SSL Example
server_ssl = smtplib.SMTP_SSL("smtp.gmail.com", 465)
server_ssl.ehlo() # optional, called by login()
server_ssl.login(gmail_user, gmail_pwd)  
# ssl server doesn't support or need tls, so don't call server_ssl.starttls() 
FROM=gmail_user
TO="[email protected]"
server_ssl.sendmail(FROM, TO, message)
#server_ssl.quit()
server_ssl.close()
