#include<stdio.h>
int w[9001];
int main()

{int max=0,i,j,k,v[9001],n,aux,ok;
float x;

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;i++) w[v[i]]=v[i];

do{
   ok=1;
   for(i=1;i<=n-1;i++)
      if(v[i]>v[i+1]) {aux=v[i];
		       v[i]=v[i+1];
		       v[i+1]=aux;
		       ok=0;}
   }
while(ok==0);

for(i=1;i<=n-1;i++)
       for(j=i+1;j<=n;j++)    {
                               x=(v[i]+v[j])/2.0;
			       if(w[x]==x) max++;
			       }

fprintf(g,"%d",max);
fclose(f);
fclose(g);
return 0;
}