import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] arr = new int[n];
		Set<Integer> set = new HashSet();
		for (int i = 0; i < n; i++) {
			arr[i] = sc.nextInt();
		}
		for (int i = 0; i < n; i++) {
			for (int j = 0; j < n && j != i; j++) {
				set.add(arr[i] + arr[j]); // 把两数之和所有结果存入HashSet，自动去重
			}
		}
		int count = 0;
		for (int i = 0; i < n; i++) {
			if (set.contains(arr[i])) count++; // 遍历每个数，只要和结果相等就加一
		}
		System.out.println(count);
	}
}

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