#include<stdio.h>

#define ll long long

int f[7006];
int v[9007],n;
ll sol;

int main ()
{
    int i,j;
    freopen("medie.in","r",stdin);
    freopen("medie.out","w",stdout);
    scanf("%d",&n);
    for(i=1;i<=n;i++)
    {
        scanf("%d",&v[i]);
        f[v[i]]++;
    }
    for(i=1;i<=n;i++)
    {
        f[v[i]]--;
        for(j=i+1;j<=n;j++)
        {
            if((v[j]+v[i])&1)
                continue;
            f[v[j]]--;
            sol+=f[(v[j]+v[i])/2];
            f[v[j]]++;
        }
        f[v[i]]++;
    }
    printf("%lld\n",sol);
    return 0;
}