/******************************************************************************

                              Online C++ Compiler.
               Code, Compile, Run and Debug C++ program online.
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*******************************************************************************/

#include <fstream>

using namespace std;
ifstream cin("medie.in");
ofstream cout("medie.out");
int n,v[9005],f[7005],i,j,cnt;
int main()
{
    cin >> n;
    for(i = 1;i <= n;i++)
    {
        cin >> v[i];
        f[v[i]]++;
    }
    for(i = 1;i <= n;i++){
        for(j =  i + 1;j <= n;j++){
            f[v[i]]--;
            f[v[j]]--;
            if(f[(v[i] + v[j]) / 2] && (v[i] + v[j]) % 2 == 0)
               cnt+=f[(v[i] + v[j])/2];
             f[v[i]]++;
             f[v[j]]++;
        }
    }
    cout << cnt;
    return 0;
}