P3131 [USACO16JAN] Subsequences Summing to Sevens S

Farmer John's $N$ cows are standing in a row, as they have a tendency to do from time to time. Each cow is labeled with a distinct integer ID number so FJ can tell them apart. FJ would like to take a photo of a contiguous group of cows but, due to a traumatic childhood incident involving the numbers $1 \ldots 6$, he only wants to take a picture of a group of cows if their IDs add up to a multiple of 7. Please help FJ determine the size of the largest group he can photograph.

The first line of input contains $N$ ($1 \leq N \leq 50,000$). The next $N$ lines each contain the $N$ integer IDs of the cows (all are in the range $0 \ldots 1,000,000$).

Please output the number of cows in the largest consecutive group whose IDs sum to a multiple of 7. If no such group exists, output 0.

In this example, $5+1+6+2+14 = 28$.