#include <stdio.h>
FILE *f, *g;
int v[9001], n, i, j, h, k=0;
int main(void)
{
f=fopen("medie.in","r");
g=fopen("medie.out","w");
fscanf(f, "%d", &n);
for (i=1;i<=n;i++)
	fscanf(f, "%d", &v[i]);
for (i=1;i<=n;i++)
	for (j=1;j<n;j++)
		for (h=j+1;h<=n;h++)
			if (j!=i&&i!=h&&v[j]+v[h]==2*v[i]&&h!=j)
				k++;
fprintf(g, "%d", k);
fcloseall();
return 0;
}