#include<stdio.h>
int main()

{int max=0,i,j,k,v[9000],n;

FILE *f=fopen("medie.in","r");
FILE *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-2;i++)
   for(j=i+1;j<=n-1;j++)
	for(k=j+1;k<=n;k++)
			     {if(v[i]==(v[j]+v[k])/2.0) max++;
			      if(v[j]==(v[i]+v[k])/2.0) max++;
			      if(v[k]==(v[i]+v[j])/2.0) max++;
			      }     

fprintf(g,"%d\n",max);

fclose(f);
fclose(g);
return 0;
}