>>237
You need to put "index.html" into the current directory:
<!DOCTYPE HTML>
<html><head><meta charset="utf-8"></head><body>
<form action="/cgi-bin/a.py" method="post">
<input type="number" name="cislo1"> + <input type="number" name="cislo2">
<input type="submit" value="oblyczyc">
</form>
</body></html>and the script into "cgi-bin/a.py".