#include <iostream>
#include <cstdio>
using namespace std;
const int VMAX = 7000;
const int NMAX = 9000;
int f[VMAX+1];
int sum[2*VMAX+1];
int v[NMAX+1];
int main()
{
    FILE *fin, *fout;
    int n,i,j,ans;
    fin=fopen("medie.in","r");
    fout=fopen("medie.out","w");
    fscanf(fin,"%d",&n);
    for(i=1;i<=n;i++){
        fscanf(fin,"%d",&v[i]);
        f[v[i]]++;
    }
    for(i=1;i<n;i++)
        for(j=i+1;j<=n;j++)
            sum[v[i]+v[j]]++;
    ans=0;
    for(i=1;i<=n;i++)
        ans+=(sum[2*v[i]]-f[v[i]]+1);
    fprintf(fout,"%d\n",ans);
    fclose(fin);
    fclose(fout);
    return 0;
}