Normal sudoku rules apply. Digits in cages cannot repeat, and sum to the total in the top left corner. Digits in red cells are the column index of the red cell's column in that row, e.g. the digit in r7c2 is the column index of the 2 in row 7, so a 6 in r7c2 means 2 is in r7c6. Not all red cells are given.