#include <cstdio>

int n, v[9005];

int main ()
{
    FILE *f = fopen ("medie.in", "r");
    fscanf (f, "%d", &n);

    int i, k, j, nr = 0;
    for (i = 1; i <= n; i++)
        fscanf (f, "%d", &v[i]);

    fclose(f);
    for (i = 1; i <= n; i++)
        for (j = 1; j < n; j++)
            for (k = j + 1; k <= n; k++)
                if (i != j && i != k)
                    if (2 * v[i] == v[j] + v[k])
                        nr++;

    f = fopen ("medie.out", "w");
    fprintf (f, "%d\n", nr);
    fclose(f);

    return 0;
}