#include<stdio.h>
int a[10000],v[10000],f[10000],n;
long long nr;
void cit(){
	freopen("medie.in","r",stdin);
	scanf("%lld",&n);
	for(int i=1;i<=n;i++)
		scanf("%lld",&a[i]);
	fclose(stdin);
}
void solve(){
	# define div2(x) ( ! ((x) & 1) )
	int i,j,aux;
	for(i=1;i<n;i++){
		f[a[i]]++;
		for(j=i+1;j<=n;j++)
			if ( div2( aux=a[i]+a[j] ) )
				++v[aux>>1];
	}
	f[a[n]]++;
}
int main(){
	cit();
	solve();
	freopen("medie.out","w",stdout);
	for(int i=1;i<=n;i++){
		if(v[a[i]]!=0)
			nr+=v[a[i]]-f[a[i]]+1;
	}
	printf("%lld",nr);
	fclose(stdout);
	return 0;
}