#include <cstdio>

#define FIN "medie.in"
#define FOUT "medie.out"

#define N 9005
#define MAX 7005

int A[MAX], v[N], n, nr;

int main()
{
    int i, j;

    freopen(FIN, "r", stdin);
    freopen(FOUT, "w", stdout);

    scanf("%d", &n);

    for (i = 1; i <= n; ++ i)
    {
        scanf("%d", &v[i]);

        ++ A[v[i]];
    }

    for (i = 1; i <= n; ++ i)
        for (j = i + 1; j <= n; ++ j)
            if (!((v[i] + v[j]) & 1))
            {
                nr += A[(v[i] + v[j]) >> 1];

                nr -= (v[i] == v[j]) ? 2 : 0;
            }

    printf("%d\n", nr);
}