#include <fstream>
#include<iostream>

using namespace std;

int a[9005], b[7005];
int main()
{
	int i, j, n, x, s=0;
    ifstream f("medie.in");
    ofstream g("medie.out");
    f>>n;
    for(i=1;i<n;i++)f>>a[i];
    for(i=1;i<=n;i++)
        for(j=i+1;j<=n;j++)
            if((a[i]+a[j])%2==0)
            {
                x=(a[i]+a[j])/2;
                b[x]++;
            }
    for(i=1;i<=n;i++)s+=b[a[i]];
    g<<s<<"\n";
}