#include <cstdio>

#define FIN "medie.in"
#define FOUT "medie.out"
#define NMAX 9001

int v[7001], rep[NMAX], N, a[NMAX];

void read ()
{
    scanf ("%d", &N);
    for (int i = 1; i <= N; ++ i)
        scanf ("%d", a + i);
}

void solve ()
{
    for (int i = 1; i < N; ++ i)
    {
        ++ rep[a[i]];
        
        for (int j = i + 1; j <= N; ++ j)
            if (!((a[i] + a[j]) & 1))
                ++ v[(a[i] + a[j])/2];
    }

    ++ rep[a[N]];

    long sol = 0;
    
    for (int i = 1; i <= N; ++ i)
        sol += long (v[a[i]] - rep[a[i]] + 1);

    printf ("%ld\n", sol);
}

int
 main ()
{
    freopen (FIN, "rt", stdin);
    freopen (FOUT, "wt", stdout);

    read ();
    solve ();

    return 0;
}