Submit Form only if Sum >= 2

[B]Description of my page:[/B]
I have a form with a number of dropdown menus.
The visitor can freely select and combine the numbers of those dropdown menus. For example he can select ‘1’ in the first dropdownd menu and ‘3’ in the second one and ‘0’ in the third. Just as he wishes. After the selection he pushes a button that submits the values to a cgi script.

[B]The problem:[/B]
Only if the sum of all dropdown menus is 2 or greater, then it is to
submit the form to the cgi script. Otherwise an error page should tell
him to revise his selection.

[B]The current Source Code:[/B]
<FORM METHOD=”post” ACTION=”http://www.domain.com/cgi-bin/file.cgi“>

<!– Dropdown Menu 1 –>
<select size=”1″ NAME=”object1″>
<option value=””>0</option>
<option value=”1″>1</option>
<option value=”2″>2</option>
<option value=”3″>3</option>

<!– Dropdown Menu 2 –>
<select size=”1″ NAME=”object2″>
<option value=””>0</option>
<option value=”1″>1</option>
<option value=”2″>2</option>
<option value=”3″>3</option>

<!– Dropdown Menu 3 –>
<select size=”1″ NAME=”object3″>
<option value=””>0</option>
<option value=”1″>1</option>
<option value=”2″>2</option>
<option value=”3″>3</option>

<!– BUTTON to submit the form to the cgi script –>
<INPUT TYPE=”image” NAME=”process_data” src=”http://www.domain.com/images/image.gif“>

</FORM>

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