#include <fstream>
#include <algorithm>
using namespace std;
int v[3015];
int main()
{
    ifstream cin ("bete2.in");
    ofstream cout ("bete2.out");
    int n,sum,s=1,e=n,mij,c=0;
    cin>>n;
    for(int i=1;i<=n;++i)
    {
        cin>>v[i];
    }
    sort(v+1,v+n+1);
    for(int i=1;i<=n;++i)
    {
        for(int j=i+1;j<=n;++j)
        {
            sum=v[j]+v[i];
            s=1,e=n;
            while(s<=e)
            {
                mij=(s+e)/2;
                if(sum>v[mij])
                {
                    s=mij+1;
                }
                else if(sum<v[mij])
                {
                    e=mij-1;
                }
                else if(sum==v[mij]) {c++;break;}
            }
        }
    }
    cout<<c;
}