#include<cstdio>
#define N 9001
#define X 7001
int n,s,a[N],r[X],v[X],i,j,x;
int main()
{FILE *f=fopen("medie.in","r"),*g=fopen("medie.out","w");
fscanf(f,"%d",&n);
for(i=1;i<=n;i++)
     fscanf(f,"%d",&a[i]),r[a[i]]++;
for(i=1;i<X;i++)
for(j=0;j<r[i];j++)
     a[++x]=i;
for(i=1;i<n;i++)
for(j=i+1;j<=n;j++)
if(!((x=a[i]+a[j])&1))
     ++v[x>>1];
for(i=1;i<=n;i++)
if(v[a[i]])
     s+=v[a[i]]-r[a[i]]+1;
fprintf(g,"%d",s);
return 0;}