Well to add the 10 and NA you will need to convert NA to an integre/number. So the following should do the job:
$var1=str_replace('NA',0,$_GET['input1']);
$var2=str_replace('NA',0,$_GET['input2']);
$var3=str_replace('NA',0,$_GET['input3']);
$var4=str_replace('NA',0,$_GET['input4']);
if (($var1+$var2)>($var3+$var4)) {
//blah
}
However you may want to replace $_GET with whatever you already have in place.
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