#include <cstdio>
double f[9005];
int main () {
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	int i,j,k,co=0,n;
	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((k!=j)&&(j!=i)&&(k!=i)&&(f[i]==(f[j]+f[k])/2)){
					++co;
				}
			}
		}
	}
	printf("%d",co);
}