#include <bits/stdc++.h>
#define ll long long
using namespace std;

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

int n,v[9005];
int main()
{

    f>>n;
    for(int i=1;i<=n;i++ )
        f>>v[i];

    int ans=0;

    for(int i=1;i<=n;i++)
        for(int j=1;j<=n;j++)
            for(int k=j+1;k<=n;k++)
                if(2*v[i]==v[j]+v[k] and i!=j  and i!=k) ans++;
    g<<ans;

    return 0;
}