#include <stdio.h>

#define Nmax 9001

int a[Nmax], nr[Nmax*2], n;
long long ret;

int main()
{
     freopen("medie.in","r",stdin);
     freopen("medie.out","w",stdout);
     
     scanf("%d", &n);
     
     for (int i=1;i<=n;++i)
          scanf("%d", &a[i]);
          
     for (int i=1;i<=n;++i)
     for (int j=i+1;j<=n;++j)
     {
          int med = (a[i]+a[j]);
          ++nr[med];
          if (med == a[i]*2) --ret;     
     }
     
     for (int i=1;i<=n;++i)
          ret += nr[a[i]*2]--;
          
     printf("%lld\n", ret);
     
     return 0;     
}