How to play Sudoku 4x4

There is really only one rule:

• Fill in the grid so that
• every row,
• every column, and
• every 4 x 4 box
• contains the digits 1 through 9 and a,b,c,d,e,f.

What is Sudoku: rules, history and terminology

This Puzzlue Address: http://sudoku.oubk.com/puzzles/5599676993.html to a friend to play together