#include <stdio.h>
#define NMax 9010 
#define VMax 7010

const char IN[]="medie.in",OUT[]="medie.out";

int N;
int a[NMax];
int b[VMax];

int solve()
{
	int ret=0,i,j;
	for (i=0;i<N;++i)
		for (j=i+1;j<N;++j)
		{
			--b[a[i]];--b[a[j]];
			if ((a[i]+a[j])%2==0)
				ret+=b[(a[i]+a[j])/2];
			++b[a[i]];++b[a[j]];
		}
	return ret;
}

int main()
{
	int i;
	freopen(IN,"r",stdin);
	scanf("%d",&N);
	for (i=0;i<N;++i)
		scanf("%d",a+i),
		++b[a[i]];
	fclose(stdin);
	
	freopen(OUT,"w",stdout);
	printf("%d\n",solve());
	fclose(stdout);
	
	return 0;
}