#include <cstdio>

using namespace std;
int v[9005];
int f[14005];
int scad[9005];
int main()
{   freopen("medie.in", "r",stdin);
    freopen("medie.out", "w",stdout);
    int n,i,j;
    long long ans=0;
    scanf("%d",&n);
    for(i=1; i<=n; i++)
        scanf("%d", &v[i]);
    for(i=1; i<=n; i++)
        for(j=i+1; j<=n; j++){
            f[v[i]+v[j]]++;
            if(v[i]==v[j]){
                scad[i]++;
                scad[j]++;
            }
        }
    for(i=1; i<=n; i++)
        ans+=f[v[i]*2]-scad[i];
    printf("%lld", ans);
    return 0;
}