Normal sudoku rules apply. Each question mark acts as a stand-in for a digit, which must be deduced for each question mark. Each circle contains 2 digits. The 2 digits must appear in at least one of the four cells surrounding that circle (if a digit repeats in a circle, it must appear twice). Digits along an arrow sum to a two-digit number. This number is a concatenation of BOTH digits in the attached circle. For clarity: A circle with the digits {1, 9} would be surrounded by at least one copy each of the digits 1 and 9 and could have arrows that summed to either 19 or 91. Its arrows could not sum to 11 nor 99, this would require a {1, 1} circle surrounded by two 1s or a {9, 9} circle surrounded by two 9s respectively.)