#!/usr/bin/env python

start = 0
seen = [0]

def get_freq_change():
    while True:
        print('reading file from start')
        with open('input.txt', 'r') as f:
            for line in f:
               yield int(line)

for change in get_freq_change():
    start += change
    if start in seen:
        break
    seen.append(start)

print(start)