Subsequences Summing to Sevens

Farmer John's $N$ cows are standing in a row. Every cow carries a distinct integer ID, which is how John tells them apart. He wants to photograph a contiguous group of cows, but because of a childhood incident involving the numbers $1$ through $6$, he only photographs a group whose IDs add up to a multiple of $7$.

Find how many cows are in the largest group John can photograph.

The first line contains $N$ ($1 \le N \le 50{,}000$). Each of the next $N$ lines contains one cow ID, in the order the cows stand. Every ID is an integer between $0$ and $1{,}000{,}000$.

Print the number of cows in the longest contiguous group whose IDs sum to a multiple of $7$. If no such group exists, print $0$.

The IDs of a long group can add up to more than a 32-bit integer holds. If you add the IDs directly, use a 64-bit type such as long long in C or C++.

In the example, $5+1+6+2+14 = 28$ is a multiple of $7$, so the answer is $5$.