#include <cstdio>
int f[7005];
int main () {
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	int i,j,k, n,c=0;
	scanf("%d",&n);
	for(i=1;i<=n;++i){
		scanf("%d",&f[i]);
	}
	for(i=1;i<=n;++i){
		for(j=1;j<=n;++j){
			for(k=j+1;k<=n;++k){
				if(i!=j&&j!=k&&i!=k){
						double aux = (f[j]+f[k]) / 2.0;
						if(aux==f[i]){
							++c;
						}
				}
				
			}
		}
	}
	printf("%d",c);
	return 0;
}