#include <cstdio>
#define Lmax 9005
#define Vmax 7005

using namespace std;

FILE *fin=freopen("medie.in","r",stdin);
FILE *fout=freopen("medie.out","w",stdout);

int a[Lmax],n;

struct element
{
   int c;
   int m;
}b[Vmax];

void citire()
{
    scanf("%d",&n);
    for(int i=0;i<n;++i)
    {
        scanf("%d",&a[i]);
        b[a[i]].c++;
        for(int j=i-1;j>=0;j--)
            if((a[i]+a[j])%2==0)
                b[(a[i]+a[j])/2].m++;
    }
}

void afisare()
{
    int s=0;
    for(int i=0;i<n;i++)
        s+=b[a[i]].m-b[a[i]].c+1;
    printf("%d\n",s);
}

int main()
{
    citire();
    afisare();
    return 0;
}