#include <iostream>
#include <cstring>
using namespace std;
int main()
{
    bool a[10001];
    int b[101],n,sum=0;
    memset(a,0,sizeof(a));
    memset(b,0,sizeof(b));
    cin>>n;
    for (int i=1;i<=n;i++)
      {
	cin>>b[i];
	a[b[i]]=1;
      }
    for (int i=1;i<=n;i++)
      for (int j=i+1;j<=n;j++)
        if (a[b[i]+b[j]]) sum++;
    cout<<sum<<endl;
    return 0;
}