#include <stdio.h>


int v[7001];



int main()
{
	int a[9001];
	register long long rez=0;
	register int aux, i, j;
	FILE *f, *g;
	f=fopen("medie.in", "r");
	g=fopen("medie.out", "w");
	int n;
	fscanf (f, "%d", &n);
	for (i=0; i<n; ++i)
		{fscanf(f, "%d", &a[i]);
		++v[a[i]];  }
	fclose(f);
	for (i=0; i<n-1; ++i)
		for (j=i+1; j<n; ++j)
			if (a[i]==a[j])
				rez+=v[a[i]]-2;
			else
				{ aux=a[i]+a[j];
				rez+= (!(aux%2)*v[aux/2]);
				}
	fprintf(g, "%lld\n", rez);
	fclose(g);
	return 0;
}