Normal sudoku rules apply. Within a cage, digits may not repeat, and the sum of their values is equal to the expression in the top left corner. There are nine vampire cells and nine prey cells in the grid, one of each in every row, column and box. Every digit from 1-9 appears exactly once in a vampire cell, and exactly once in a prey cell. A vampire cell steals the value of the digit in the prey cell in the same box and adds it to its own digit to create its value. The value of the digit in a prey cell is 0.