import java.util.Scanner;


public class Main {
	public static void main(String[] a) {
		Scanner s=new Scanner(System.in);
		s.nextInt();
		int[] I=new int[11];
		int U=0,D=0,i;
		while(s.hasNext()){
			if(I[i=s.nextInt()]<0)U++;else D++;
			I[i]=~I[i];
		}
		System.out.println(U+(D-U)/4);
	}
}