Normal sudoku rules apply. Colour the x'th cell in each row red (where x is the digit in column 1 of that row) and colour the y'th cell in each column blue (where y is the digit in row 1 of that column). Every row, column and box must contain exactly one red and one blue cell, and red and blue cells may not overlap. Clues outside the grid indicate the sum of digits sandwiched between the red and blue cell in that row/column.