#include <fstream>
using namespace std;
ofstream out ("medie.out");
ifstream in ("medie.in");
const int N=7004;
int f[N],s[N];
int main()
{
    int n,x,i,j,mx=0,sum=0;
    in>>n;
    for (i=1;i<=n;i++)
    {
        in>>x;
        f[x]++;
        mx=max(mx,x);
    }
    mx*=2;
    for (i=2;i<=mx;i+=2)
    {
        s[i]+=max((f[i/2]*(f[i/2]-1))/2,0);
        for (j=i/2-1;j>0;j--)
            s[i]+=f[j]*f[i-j];
    }
    mx/=2;
    for (i=1;i<=mx;i++)
        sum+=(s[i*2]-f[i]+1)*f[i];
    out<<sum;
    return 0;
}