Normal sudoku rules apply. All the 2x2 areas of the grid, when read in normal reading order, make a total of 64 four-digit numbers. Clues in the centre of a 2x2 area show its rank among those 64 numbers, where the lowest is 1 and the highest is 64. The two “X”s are the same number. One or more equal numbers are considered to be at rank N when there are N-1 numbers of strictly lower rank. (Eg, if there are three equal numbers which have 19 numbers lower than them then these three numbers would be rank 20 and the next rank will be 23, i.e. the ranks could go ⋯, 19, 20, 20, 20, 23, ⋯)