#include<stdio.h>
int modul(int x){
	if(x<0)
		return -x;
	return x;
}
int main(){
	int n,v[9000],i,j,k,s=0;
	freopen("medie.in","r",stdin);
	freopen("medie.out","w",stdout);
	scanf("%d",&n);
	for(i=0;i<n;++i)
		scanf("%d",&v[i]);
	for(i=0;i<n;++i)
		for(j=0;j<n;++j)
			for(k=j+1;k<n;++k)
				if(v[i]-v[j]==v[k]-v[i]&&k!=i&&j!=i)
					s++;
	printf("%d\n",s);
	fclose(stdin);
	fclose(stdout);
	return 0;
}