flow.size: 6
row.1: 0,0,0,0,0,0
row.2: 0,3,0,0,3,0
row.3: 0,0,0,0,6,0
row.4: 0,0,4,2,0,0
row.5: 0,0,0,0,4,6
row.6: 2,0,0,0,0,0

