#include <stdio.h>
#define Nmax 9005
#define Vmax 7005

int v[Nmax],cnt[Vmax];
int n,i,j,nr;

int main(){
	freopen("medie.in","r",stdin);
   freopen("medie.out","w",stdout);
   scanf("%d",&n);
   for(i=1;i<=n;++i) scanf("%d",&v[i]), cnt[v[i]]++;

   for(i=1;i<n;++i)
     for(j=i+1;j<=n;++j)
     		if(((v[i]+v[j]) & 1) ==0){
         	nr += cnt[(v[i]+v[j])/2];
            if(v[i]==v[j]) nr -=2;
         }

   printf("%d\n",nr);
   fclose(stdin); fclose(stdout);
   return 0;
}