#include <iostream>

using namespace std;

int a[9010],medie[7010],apar[7010];

int main()
{
    freopen ("medie.in","r",stdin);
    freopen ("medie.out","w",stdout);

    int n,i,j,x;
    long sol=0;
    scanf("%d",&n);
    for(i=1;i<=n;i++)
        scanf("%d",&a[i]);
    for(i=1;i<n;i++)
    {
        apar[a[i]]++;
        for(j=i+1;j<=n;j++)
        {
            x=a[i]+a[i];
            if(!(x&1))
                ++medie[x>>1];
        }
    }
    apar[a[n]]++;
    for(i=1;i<=n;i++)
        if(medie[a[i]])
            sol+=long (medie[a[i]] - apar[a[i]] +1);
    printf("%ld\n",sol);
    return 0;
}