#include <cstdio>
#include <string>
#include <cstdlib>
#define maxn 9001
int main()
{
	int n, v[maxn], h[maxn], i,j;
	memset(v, 0, sizeof(v));
	memset(h, 0, sizeof(h));
	
	freopen("medie.in", "r", stdin);
	freopen("medie.out", "w", stdout);
	scanf("%d\n", &n);

	for(i=1;i<=n;++i) scanf("%d\n", v+i);
	
	for(i=1;i<=n;++i) ++h[v[i]];
//	for(i=1;i<=10;++i) printf("%d ", h[i]);
	//printf("\n");
	
	int nr=0;
	for(i=1;i<n;++i)
		for(j=i+1;j<=n;++j)
		{
			//printf("%d %d %d %d\n", v[i], v[j], (v[i]+v[j])>>1, h[(v[i]+v[j])>>1]);
			if(v[i]==v[j]) nr+=h[v[i]]-2;
				else if(((v[i]+v[j])>>1)*2==(v[i]+v[j]))nr+=h[(v[i]+v[j])>>1];
		}
	printf("%d\n", nr);
return 0;
}