Here is a coding problem:
<PROBLEM>

Here is an attempted solution:
<SOLUTION>

Here are several unit tests (including the input, the execution output, and the predicted correctness of the output)
<UNIT_TESTS>

Rate the difficulty of the programming problem on a scale from 1 to 10 based on the problem description, the attempted solution, and the unit test evalution results.
1 indicates the probelm is very easy to solve, while 10 indicates the problem is very hard to solve.
Rate the difficulty of the programming problem on a scale from 1 to 10. Just respond immediately with the number.
