#include <fstream>

using namespace std;
ifstream in("medie.in");
ofstream out("medie.out");
int a[9002],frec[9002];
int main()
{int n,i,j,m,sum=0;
in>>n;
for(i=1;i<=n;i++)
{
    in>>a[i];
    frec[a[i]]++;
}
for(i=1;i<=n;i++)
{
    for(j=i+1;j<=n;j++)
    {
        if((a[i]+a[j])%2==0)
        {
            m=(a[i]+a[j])/2;
            if(m==a[i])
                sum--;
            if(m==a[j])
                sum--;
            sum=sum+frec[m];
        }
    }
}
out<<sum<<'\n';
    return 0;
}