#include <stdio.h>

#define MAX_N 20010

typedef long long llong;

int N, V[MAX_N], cnt[MAX_N], B[MAX_N];
llong res;

int main(void)
{
    freopen("medie.in", "rt", stdin);
    freopen("medie.out", "wt", stdout);

    int i, j, k;

    scanf("%d\n", &N);

    for(i = 1; i <= N; i++)
        scanf("%d\n", &V[i]), B[V[i]]++;

    for(i = 1; i <= N; i++)
     for(j = i+1; j <= N; j++)
        cnt[V[i]+V[j]]++;

    for(i = 1; i <= N; i++)
        res += (llong)cnt[2*V[i]]-(llong)(B[V[i]]-1);

    printf("%lld\n", res);

    return 0;
}