#include<fstream>

using namespace std;

ifstream f("medie.in");
ofstream g("medie.out");

const int maxn=9003;
const int maxv=6004;

int n,a[maxn],frec[maxv],i,j,k,r;

int main()
{
    f>>n;
    for(i=1;i<=n;++i)
        f>>a[i],++frec[a[i]];
    for(i=1;i<=n;++i)
        for(j=i+1;j<=n;++j)
        {
            r=a[i]+a[j];
            if(r&1)
                continue;
            r>>=1;
            k+=frec[r];
            if(a[i]==r)
                k-=2;
        }

    g<<k<<"\n";

    f.close();
    g.close();

    return 0;
}