#include <stdio.h>

using namespace std;

int vc[7002],v[9002];

int main()
{
    freopen("medie.in","r",stdin);
    freopen("medie.out","w",stdout);
    int n,i,j,ct=0,s;
    scanf("%d",&n);
    for(i=1;i<=n;i++)
    {
        scanf("%d",&v[i]);
        vc[v[i]]++;
    }
    for(i=1;i<=n;i++)
    {
        for(j=i+1;j<=n;j++)
        {
            s=v[i]+v[j];
            if(s%2==0)
            {
                s=s/2;
                ct+=vc[s];
                if(v[i]==v[j] && vc[s]!=0)
                {
                    ct-=2;
                }
            }
        }
    }
    printf("%d",ct);

    return 0;
}