#include <fstream.h>
int main()
{ int  v[7000],n,i,j,ok,aux,k=0,ma;
ifstream f ("medie.in");
ofstream g ("medie.out");
f>>n;
for (i=1;i<=n;i++) f>>v[i];
do { ok=0;
    for (i=1;i<=n-1;i++) {
	 aux=v[i];
	 v[i]=v[i+1];
	 v[i+1]=aux; ok=1; }
    } while (ok==1);

for (i=1;i<=n;i++) {
	 for (j=1;j<=n;j++) {         
	       if (v[i]!=v[j]){ for (k=j+1;k<=n;k++) {
			             if (v[i]!=v[k]) { ma=(v[j]+v[k])/2;
					    if (ma==v[i]) k++; }}}} }
g<<k;
f.close ();
g.close ();
}