Sudoku, and Kakuro

During our trip in Switzerland, we spent a lot of time in train, so usually I’m not so interested in puzzle games but I tried to do some games on papers.
Sudoku is very popular now. The rule is very simple, but not easy. All we have to do is put numbers from 1 to 9 in 9x9 boxes containing nine 3x3 boxes, avoiding put the same number in one line and one 3x3 box. It looks not difficult if you know the rule, but actually it’s a bit difficult.

Kakuro is more complicated. I think for beginners it should be more difficult than Sudoku, but it has some kind of formula, so if you learn them it should be apparently simpler.