Subsequences Summing to Sevens

```
import sys
sys.stdin = open("div7.in")
sys.stdout = open("div7.out", "w")
n = int(input())
cow_ids = []
printed = False
for i in range(n):
cow_ids.append(int(input()))
pref_sums = [0]
for cow_id in cow_ids:
pref_sums.append(pref_sums[-1] + cow_id)
for i in range(n, 0, -1):
starts_max = n - i
for j in range(0, starts_max + 1):
sum_of_ids = pref_sums[j + i] - pref_sums[j]
if sum_of_ids % 7 == 0:
print(i)
printed = True
if printed == True:
break
if not printed:
print(0)
```

The following code is O(n^2) and only passes half of the tc. Any suggestions on how to improve this?