This program is going to produce results, and n reaches 500,000, which is beyond the expression range of integers, and of course it overflows.

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A word of advice: A letter book is better than no book, especially now some computer books compiled by China people are not rubbish.

In addition, learn to debug by yourself. Even if you can't use the vb debugger, you can display the values of some variables in the label control to observe the changes.