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#include <bits/stdc++.h>
using namespace std;
//这题30分的一般都是没有去重……1+4=5和2+3=5算同一个……
//这道题还是挺有意思的，注意，前方有坑！需要理解 5=1+4=2+3，但5只算一次的道理。
const int N = 110;
int n;
int a[N];
bool b[N]; //标识已使用
int cnt;
int main() {
cin >> n;
for (int i = 1; i <= n; i++)cin >> a[i];
//暴力三层循环+标识数组
for (int i = 1; i < n; i++)
for (int j = i + 1; j <= n; j++)
for (int k = 1; k <= n; k++)
if (a[k] == a[i] + a[j] && !b[k]) cnt++, b[k] = true;
printf("%d", cnt);
return 0;
}
