#include <cstdio>

using namespace std;

short v[9192], nr[8192], x[9192<<1];

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

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

  scanf("%d",&n);

  for(i=0;i<n;++i)
   {
    scanf("%d",x+i);
    v[i] = x[i] >> 1;
   }

  for(i=0;i<n;++i)
   for(j=i+1;j<n;++j)
    {
     tmp = v[i]+v[j];
     sol -=(tmp == x[i]);
     ++nr[tmp];
   }

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

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

  return 0;
 }