#include <cstdio>

using namespace std;

long v[9010],x[7010],y[7010],i,j,n,s;

int main(){
freopen("medie.in","r",stdin);
freopen("medie.out","w",stdout);
scanf("%ld",&n);
for (i=1;i<=n;i++)
    {
    scanf("%d",&v[i]);
    x[v[i]]++;    
    }
for (i=1;i<n;i++)
    for (j=i+1;j<=n;j++)
        {
        if (((v[i]+v[j])>>1)<<1==v[i]+v[j])
            y[(v[i]+v[j])>>1]++;
        }
for (i=1;i<=n;i++)
    {
    if (y[v[i]])
        s+=y[v[i]]-x[v[i]]+1; 
    }
printf("%ld",s);
return 0;    
}