#include <cstdio>
#define deb(n) fprintf(stderr,"%d ",(n));
#define DN 9005

int n,v[DN],aparitii[DN];

int main()
{
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	int i;
	scanf("%d",&n );
	for(i=1; i<=n;++i) {
	    scanf("%d",&v[i] );
	    ++aparitii[v[i]];
	}
	int rez=0;
	for(i=2; i<=n;++i) for(int j=1; j<i; ++j) {
	    int sum=v[i]+v[j];
	    if(0==sum%2){
	        rez+=aparitii[sum/2];
	        if(v[i]==v[j]) rez-=2;
	    }
	}
	printf("%d",rez);
	return 0;
}