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

                              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()
{
    ios_base::sync_with_stdio();
    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++){
            if(v[i] == v[j]){
                f[v[i]] -= 2;
                cnt += f[v[i]];
                f[v[i]] += 2;
            }
            else if(v[i] % 2 == v[j] % 2)
               cnt+=f[(v[i] + v[j])/2];
            
        }
    }
    cout << cnt;
    return 0;
}