#include<stdio.h>
#define dim 9000
int a[dim],n,m,i,k,sol,j;
void read()
{
     scanf("%d",&n);
     for(i=1;i<=n;i++)
     scanf("%d",&a[i]);
}
void solve()
{
    for(i=1;i<=n;i++)
    {
        for(k=1;k<n;k++)
        {
            if(k==i)
            continue;
            for(j=k+1;j<=n;j++)
            {
                if(i==j)
                continue;
                if(a[i]==(float)(a[j]+a[k])/2)
           {
                 sol++;
             //    printf("%d %d %d\n",a[i],a[k],a[j]);
                 }
            }}}}

 
int main ()
{
    freopen("medie.in","r",stdin);
    freopen("medie.out","w",stdout);
    read();
    solve();
    printf("%d",sol);
    return 0;
}