problem with balance

balance= float((raw_input('Enter the outstanding balance on your credit card: ')))
annualinterestrate= float((raw_input('Enter the credit interest as a decimal: ')))
minmonthlypaymentrate= float((raw_input('Enter the minimum monthly payment rate as a decimal: ')))

Monthlyinterestrate = annualinterestrate/12
nummonths = 1
totalamtpaid = 0

while nummonths <= 12:
    minmonthlypayment = round(minmonthlypaymentrate * balance,2)
    interestpaid = round(Monthlyinterestrate * balance,2)
    principalpaid = (minmonthlypaymentrate - interestpaid
    balance = principalpaid

print('month:',nummonths)
print('minimum monthly payment:',minpayment)
print('remaining balance:',balance)

nummonths+= 1