The BWS Loteria draw is held once a year. N people enter it, and each of them picks K numbers. Write Bi,j for the j-th number picked by the i-th person. The organizers then pick K positive integers of their own, called W1,W2,…,WK.
The winners are decided like this.
- A non-empty subset of the N entrants is drawn at random.
- Add up the first number picked by every person in that subset and call the total S1. That is, S1 is the sum of Bi,1 over the indices i in the subset. Compute S2,…,SK the same way.
- For every j, check whether Wj and Sj have the same parity, meaning both are even or both are odd.
- If the parities agree for every j, that set of people wins.
The organizers want to know whether they can pick W1,W2,…,WK so that no subset of the entrants wins.