
Please apply the following process until the same results is obtained to solve "sum of natural number between 3 and 19".
1. Propose 2 different solutions.
2.Based on the 2 solutions, solve problem A and obtain 2 results.
3. If the 2 results are the same, return the results and exit, else think step by step why the results are different.
4. Based on the reason, propose 2 improved solutions.
5. Go to 2.