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You must set your post counter outside your loop first before it will work. If you turn debugging on, you will get an undefined variable notice. You can do something like this just outside your loop before the loop starts

$postvariable = 1;

Alternatively you can make use of the build-in post counter, $wp_query->current_post. It starts counting at 0, so to target the firts post, you can do

if( 0 == $wp_query->current_post ) {
    //do something for first post
}