#include<stdio.h>
#include<algorithm>
#define ll long long
using namespace std;

const int N=9001;

int n,v[N],c[N];

long long rez;

void read()
{
	scanf("%d",&n);
	for( int i=1 ; i<=n ; ++i )
	{
		scanf("%d",&c[i]);
		++v[c[i]];
	}
	sort(c+1,c+n+1);
}

void solve()
{
	int i,j,t;
	for (i=1; i<n; i++)
		for (j=i+1; j<=n && 2*c[j]-c[i]<=c[n]; j++)
		{
			t=2*c[j]-c[i];
			if (c[i]!=c[j])
				rez+=v[t];
			else
				rez+=v[t]-2;
		}
	printf("%lld\n",rez);
}

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