<?php
    $input_lines = trim(fgets(STDIN));
    $input_lines = str_replace(array("\r\n","\r","\n"), "", $input_lines);
    do {
        $tmpStr[] = $input_lines;
        $input_lines = trim(fgets(STDIN));
        $input_lines = str_replace(array("\r\n","\r","\n"), "", $input_lines);
    } while ($input_lines !== "");
    
    $items = [1  => 0,
    		  2  => 0,
    		  3  => 0,
    		  4  => 0,
    		  5  => 0,
    		  6  => 0,
    		  7  => 0,
    		  8  => 0,
    		  9  => 0,
    		  10 => 0];
    array_shift($tmpStr);
	
    foreach($tmpStr as $value){
    	$s = explode(" ", $value);
    	foreach($s as $value2){
    		$items[$value2] += 1;
    	}
    }
    
    $power = 0;
    $tmpCnt = 0;
    foreach($items as $key => $value){
    	$power += floor($value / 2);
    	$tmpCnt += ($value - floor($value / 2) * 2);
    } 
    echo $power + floor($tmpCnt / 4);