#include <cstdio>

using namespace std;

int x[10000],y[15000];

int main()
{
    int n,i,j,ok,nr,sum;
    FILE *f=fopen("medie.in","r");
    FILE *g=fopen("medie.out","w");
    fscanf(f,"%d",&n);
    for(i=1; i<=n; ++i)
    {
        fscanf(f,"%d",&x[i]);
        ++y[x[i]];
    }
    sum=0;
    for(i=1;i<=7000;++i)
    {
        sum+=y[i]*(y[i]-1)*(y[i]-2)/2;
        for(j=1;j<i;++j)
        {
            sum+=y[j]*y[2*i-j];
        }

    }
    fprintf(g,"%d",sum);
    return 0;
}