B. Little C Loves 3 II
brute force; constructive algorithms; flows; graph matchings
Little C loves number «3» very much. He loves all things about it.Now he is playing a game on a chessboard of size \(n \times m\). The cell in the \(x\)-th row and in the \(y\)-th column is called \((x,y)\). Initially, The chessboard is empty. Each time, he places two chessmen on two different empty cells, the Manhattan distance between which is exactly \(3\). The Manhattan distance between two cells \((x_i,y_i)\) and \((x_j,y_j)\) is defined as \(|x_i-x_j|+|y_i-y_j|\).He want to place as many chessmen as possible on the chessboard. Please help him find the maximum number of chessmen he can place.
A single line contains two integers \(n\) and \(m\) (\(1 \leq n,m \leq 10^9\)) — the number of rows and the number of columns of the chessboard.
Print one integer — the maximum number of chessmen Little C can place.
In the first example, the Manhattan distance between any two cells is smaller than \(3\), so the answer is \(0\).In the second example, a possible solution is \((1,1)(3,2)\), \((1,2)(3,3)\), \((2,1)(1,3)\), \((3,1)(2,3)\).