#include<cstdio>
using namespace std;

FILE *fin = freopen("medie.in","r",stdin);
FILE *fout = freopen("medie.out","w",stdout);

int N, ans;
int v[9001], w[7002];
long long Sol;

int main()
    {
        scanf("%d", &N);

        for (int i = 1; i <= N; i++)
            scanf("%d", &v[i]), w[v[i]] ++;

        for (int i = 1; i < N; i++)
            for (int j = i + 1; j <= N; j++)
                if (!((v[i] + v[j]) % 2))
                {
                    ans = (v[i] + v[j])/2;
                    Sol += w[ans];

                    if (v[i] == v[j])
                        Sol -= 2;
                }

        printf("%lld", Sol);
    }