#include<stdio.h>
int main()
{
long v[10000],i,j,k,sol=0,n;
FILE*f=fopen("medie.in","r");
FILE*g=fopen("medie.out","w");
fscanf(f,"%ld",&n);
for(i=1;i<=n;++i) fscanf(f,"%ld",&v[i]);
for(i=1;i<=n;++i)
	{
	for(k=1;k<=n;++k)
		for(j=1;j<k;j++)
			if (i!=j&&i!=k&&v[i]==(v[j]+v[k])/2) sol++;
	}
fprintf(g,"%ld",sol);
fclose(f);
fclose(g);
return 0;}