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.
from tasks import  send_mail_task
from string import Template
from services.email.mail_template.email_template import _template
from dotenv import load_dotenv
import os

load_dotenv()

# from celery import Celery
name="Nguyen Quoc Khoa"
_email_template =_template
new_html=_email_template.substitute(ticket_uri="https://app.inz-dev.esollabs.com/qrResult?token_id=0&contract=0x081cc3cb8f5d5beb8e3865d5d46b91921c5f2819&fbclid=IwAR0Ivwlm36h48NiDyp4e0ZITRcIT1bc-8dGWof3lAGJWVjILYaKOIf-RBRM")
subject="hello from Khoa"
# print(new_html)
sender_account=os.getenv("sender_account")
print(sender_account)
sender_pwd=os.getenv("sender_pwd")
email_port=os.getenv("email_port")
send_mail_task.delay(sender_account=sender_account,sender_pwd=sender_pwd, receiver_email="[email protected]",subject=subject,message=new_html,qr_code='https://app.inz-dev.esollabs.com/qrResult?token_id=0&contract=0x081cc3cb8f5d5beb8e3865d5d46b91921c5f2819&fbclid=IwAR0Ivwlm36h48NiDyp4e0ZITRcIT1bc-8dGWof3lAGJWVjILYaKOIf-RBRM')
