import java.util.Scanner; public class Main {

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);

    //定义输入数字的个数
    int n = sc.nextInt();

    //定义存储随机数的数组
    int [] arr = new int[n];
    for (int i = 0; i < n; i++) {
        arr[i] = sc.nextInt();
    }

    //把数组从小到大排列
for (int i = 0; i < n; i++) {
        for (int j = i + 1; j < n; j++) {
            if (arr[i] > arr[j]) {
                int temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
        }
    }

    //定义boolean【】  usedArr数组，用来标记被用过的k，因为题目是说】求多少个k满足条件
    boolean[] usedArr = new boolean[n];

    //用count计算有几组题目要求的数
    int count = 0;
    for (int i = 0; i <= n - 3; i++) {
        for (int j = i + 1; j <= n - 2; j++) {
            for (int k = j + 1; k <= n - 1; k++) {
                if (usedArr[k] == false && (arr [i] + arr[j] ==arr[k])){
                    count++;
                    usedArr[k] = true;
                }
            }
        }
    }

    System.out.println(count);
}

}