#include<stdio.h>
#define N 10000
int n,a[N],v[N],f[N];
void read()
{
	int i;
	scanf("%d",&n);
	for(i=0;i<n;i++)
	{
		scanf("%d",&a[i]);
		f[a[i]]++;
	}
}
void solve()
{
	int i,j,k,aux,nr=0;
	for(i=0;i<n;i++)
	{
		aux=a[i];
		for(j=0;j<n-1;j++)
			if(j!=i)
				for(k=j+1;k<n;k++)
					if(2*aux==a[j]+a[k]&&k!=i)
						nr++;
	}
	printf("%d\n",nr);
}
int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	read();
	solve();
	return 0;
}