#include <cstdio>

using namespace std;

int v[9192], nr[8192<<1];

int main()
 {
  freopen("medie.in","r",stdin);
  freopen("medie.out","w",stdout);

  int i,j,n,tmp;
  long long sol = 0;

  scanf("%d",&n);
  
  for(i=0;i<n;++i)
   scanf("%d",v+i);
   
  for(i=0;i<n;++i)
   for(j=i+1;j<n;++j)
    {
     tmp = v[i]+v[j];
     sol -=(tmp == v[i]<<1);
     ++nr[tmp];
   }

  for(i=0;i<n;++i)
   sol += nr[v[i]<<1]--;

  printf("%lld\n",sol);

  return 0;
 }