Incorrectly calculating bounce rates

site.py

from collections import defaultdict

initial_pages = defaultdict(lambda: 0)
bounce_rates = defaultdict(lambda: 0)


for line in open('site_data.csv'):
    first, second = line.rstrip().split(',')
    if first == '-1':
        initial_pages[second] += 1

    if second == 'B':
        bounce_rates[first] += 1


print('Initial states:')
total_pages = sum(initial_pages.values())
initial_page_probabilities = {key: value/total_pages for key, value in initial_pages.items()}
for page, probability in initial_page_probabilities.items():
    print(f'Page {page}: {probability}')

print('Bounce rates:')
total_bounces = sum(bounce_rates.values())
bounce_rate_probabilities = {key: value/total_bounces for key, value in bounce_rates.items()}
for page, probability in bounce_rate_probabilities.items():
    print(f'Page {page} bounce rate: {probability}')