Vending Machine Program: Problem with variables

[icabero0225]

I'm trying to make a simple vending machine program. Running this program gives me this error:

Error:
Traceback (most recent call last):
  File "C:/Users/ignacio cabero/Desktop/Python/vendingmachine.py", line 13, in <module>
   insertp25()
 File "C:/Users/ignacio cabero/Desktop/Python/vendingmachine.py", line 11, in insertp25
   total_insert += 0.25
UnboundLocalError: local variable 'total_insert' referenced before assignment

global total_cost, total_insert
total_cost=0
total_insert=0

float(total_cost)
float(total_insert)
def insertp25():
   total_insert += total_insert + 0.25
   print(total_insert)
insertp25()
insertp25()

What can I do? I made the variables global yet nothing seems to work.