Group Sum Sudoku

Group Sum Sudoku is a sudoku variant with four variants of its own. In Group Sum Sudoku, you must place a digit from 1-9 in each row, column and 3x3 box only once. In addition, the four digits surrounding a circle must add up to the number inside that circle. Come here for Group Sum Sudoku's variants:
 * Diagonal Sum Sudoku
 * Colour Regions Sum Sudoku
 * Extra-Regions Sum Sudoku
 * Colour Sum SudokuGroup_Sum_Sudoku.png