void cal_disw() {
memset(disw, -1, sizeof(disw));
deque<Pos> sq(0);
for(int i = 0; i < wk.size(); ++i) {
disw[wk[i].x][wk[i].y] = 0;
sq.push_back(wk[i]);
}
while(!sq.empty()) {
Pos p = sq.front();
sq.pop_front();
int now = disw[p.x][p.y];
for(int d = 0; d < 4; ++d) {
p.x += dx[d];
p.y += dy[d];
if(p.inside(n) && disw[p.x][p.y] == -1) {
disw[p.x][p.y] = now + 1;
sq.push_back(p);
}
p.x -= dx[d];
p.y -= dy[d];
}
}
}
