201403-1 相反数

#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;

const int MAXN = 510;

int n, a[MAXN];
int cnt[MAXN];

int main() {
	scanf("%d", &n);
	for (int i = 0; i < n; ++i) { 
		scanf("%d", &a[i]);
		cnt[abs(a[i])]++;
	}
	
	int res = 0;
	for (int i = 0; i < n; ++i)
		if (cnt[abs(a[i])] == 2) res++;
	res /= 2;
	
	printf("%d", res);
	
	return 0;
}