import java.util.Arrays;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int[] nums = new int[n];
        int cnt = 0;
        // 排序
        Arrays.sort(nums);
        for (int i = 0; i < n; i++) {
            nums[i] = scanner.nextInt();
        }
        // 从最后一个数开始
        for (int i = n-1; i >= 2; --i) {
            for (int j = 0; j < i; j++) {
                for (int k = j+1; k < i; k++) {
                    if(nums[i]==nums[j]+nums[k]){
                        nums[i] = 0;
                        ++cnt;
                    }
                }
            }
        }
        System.out.println(cnt);
    }
}