# include <stdio.h>

# define  maxn	9009
# define  maxv  10000

# define  _fin  "medie.in"
# define  _fout "medie.out"


int cv[maxv], a[maxn], n, i, j, sol;

int main()
{
	freopen(_fin, "r", stdin);
	freopen(_fout,"w", stdout);
	
	for (scanf("%d", &n), i=1; i<=n; ++i) scanf("%d", a+i), ++cv[a[i]];
	
	for (i=1; i<n; ++i)
		for (j=i+1; j<=n; ++j)
			if ( !((a[i]+a[j])&1) ) sol += ( cv[(a[i]+a[j])>>1] - ((a[i]==a[j])<<1) );
	
	printf("%d\n", sol);
	
	return 0;
}