#include<stdio.h>

int n,v[10006],fe[10006],fm[10006],i,j;

int main(){


FILE *f=fopen("medie.in","r");
fscanf(f,"%d",&n);

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

int q;

  for(i=1;i<n;i++){
    for(j=i+1;j<=n;j++){
    q=v[i]+v[j];
       if((q&1)==0){
       fm[q>>1]++;
       }
    }
  }

fclose(f);

int rez=0;

  for(i=1;i<=n;i++){
    if(fm[v[i]])
    rez+=fm[v[i]]-(fe[v[i]]-1);
  }

FILE *g=fopen("medie.out","w");
fprintf(g,"%d",rez);
fclose(g);

return 0;
}