#include<stdio.h>

const int N=9001;

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

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

void solve()
{
	for( int i=1 ; i<=n ; ++i )
		for( int j=i+1 ; j<=n ; ++j )
			if( j!=i )
			{
				if(c[i]==c[j])
					rez+=v[c[i]]-2;
				else
					rez+=v[ c[i]*2-c[j] ];
			}
}

int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	read();
	solve();
	printf("%d\n",rez);
	return 0;
}