//#include<fstream.h>
#include<stdio.h>
int v[9001];
int w[7001];
//unsigned long cont;
long cont;
int main()
{int n,x,i,j,ok;
// ifstream f("medie.in");
// ofstream g("medie.out");
// f>>n;
 FILE *f=fopen("medie.in","r");
 FILE *g=fopen("medie.out","w");
 fscanf(f,"%d",&n);
 for(i=1;i<=n;i++)
  {//f>>x;
   fscanf(f,"%d",&x);
   v[i]=x;
   w[x]++;
  }
// f.close();
 cont=0;
 for(i=1;i<=n;i++)
  for(j=(i+1);j<=n;j++)
   {ok=v[i]+v[j];
    if(ok%2==0)
      {ok=ok/2;
       if((ok!=v[i])&&(ok!=v[j])) cont=cont+w[ok];
	 else if((ok==v[i])&&(ok==v[j])) cont=cont+w[ok]-2;
	  else cont=cont+w[ok]-1;
      }
   }
// g<<cont<<'\n';
// g.close();
 fprintf(g,"%d",cont);
 fprintf(g,"\n");
return 0;
}
/*
#include<stdio.h>
#include<stdlib.h>
int n,a[100];
int main()
{FILE *f=fopen("xx.in","r");
 FILE *g=fopen("xx.out","w");
 fscanf(f,"%d",&n);
 int i;
 for(i=1;i<=n;i++)
  {fscanf(f,"%d",&a[i]);
  }
 fprintf(g,"%d \n",n);
 for(i=1;i<=n;i++)
  {fprintf(g,"%d ",a[i]);
  }
 fprintf(g,"\n");
return 0;
}
*/