#include<stdio.h>
#include<iostream.h>
int v[9001];
FILE *f=fopen("medie.in","r");
FILE *g=fopen("medie.out","w");

void citire(int v[9001], int n)
{for (int i=1;i<=n;i++) fscanf (f,"%d",&v[i]);
}

int main()
{int n, i, k=0, j, l;
 float ma;
 fscanf (f,"%d",&n);
 citire(v,n);

 for (i=1;i<=n;i++) for (j=1;j<=n;j++) for (l=j+1;l<=n;l++)
	      if (i!=j&&i!=l&&j!=l) {ma=(float)(v[j]+v[l])/2;
				     if (v[i]==ma) k++;
				     }
 fprintf(g,"%d",k);
 fclose(f);
 fclose(g);
 return 0;
}