#include<cstdio>

void cit(int& din,double a[]) {
	int i;
	scanf("%d", &din);
	for(i=1;i<=din;++i) 
		scanf("%lf", &a[i]);
}
 void swap(double &a,double &b){
	 double c;
	 c=a;
	 a=b;
	 b=c;
 }
int main(){
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	int n,i,j,k,contor=0;
	double v[1005];
	cit(n,v);
	for(i=1;i<=n;++i)
		for(j=1;j<n;++j)
			for(k=j+1;k<=n;++k)
		if(i!=j && i!=k && k!=j) {
			double x = (v[j]+v[k])/2;
			if(x==v[i])
		      contor++;
		}
	printf("%d",contor);
	return 0;
}