SUDOKU

Write a number from 1 to 9 into each empty square in the grid so that every number from 1 to 9 appears exactly once in each horizontal row, each vertical column, and each 3×3 block.
3    8    4    
  6    1    5  
    7    4    8
2    5    3    
  9          2  
    5    9    4
8    4    7    
  4    3    6  
    1    6      
**** Very hard