Normal sudoku rules apply. A border square is a 2x2 square that crosses at least one 3x3 box border. For all border squares exactly 2 digits are the same. The middle 3x3 box in the grid is a magic square i.e. the sum of digits in each row, column and main 3-cell diagonal in that box is N (where N must be determined by the solver). There are exactly N ‘friendly’ cells in the grid. A cell is friendly if its digit is equal to at least one of its row number, column number or box number. (Rows are numbered top to bottom; columns are numbered left-to-right; boxes are numbered in normal reading order.) Digits separated by a black dot must be in ratio 1:2 (ie one digit is double the other). Not all dots are necessarily given.