#include <stdio.h>
#include <vector>
using namespace std;

#define hg 8192

char ch[hg];
int poz;
int N;
int rez , i, j;
int V[10001], res[10001];

FILE *f=fopen("medie.in","r");
FILE *g=fopen("medie.out","w");

inline void cit (int &x)
{
    x = 0;
    if (ch[0]=='\0') fread (ch, 1, hg, f);
    else while (ch[poz] < '0' || ch[poz] > '9')
            if (++poz == hg)
                fread (ch, 1, hg, f), poz = 0;

    while (ch[poz] >= '0' && ch[poz] <= '9')
    {
        x = x * 10 + ch[poz] - '0';
        if (++poz == hg)
            fread(ch, 1, hg, f), poz = 0;
    }
}

int main()
{
    cit(N);

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

    for (i = 1; i <= N; i++)
        for (j = i + 1; j <= N; j++)
        {
            int aux = V[i] + V[j];
            res[aux]++;
            if (aux == V[i] << 1)
                rez--;
        }

    for (int i=1;i<=N;++i)
    {
        rez += res[V[i] << 1];
        res [V[i] << 1]--;
    }

    fprintf(g,"%d",rez);

    return 0;
}