#include<stdio.h>
#define max 3000
int x,m,nr=0,n,k,b,a,v[max+1],in[max];
void aranjare()
{int i,aux,j;
     for(i=1;i<x;i++)
       for(j=i+1;j<=x;j++)
	   if(v[i]>v[j])
	     {
			aux=v[i];
			v[i]=v[j];
			v[j]=aux;
	     }
}
int caut(int m,int k)
{ int lo=1,i,j,hi=x,mid;
while(lo<=hi)
       {
	 mid=lo+(hi-lo)/2;
	 if(v[mid]==k+m) ++nr,lo=mid+1;
	 else if(m+k<v[mid]) hi=mid-1;
	 else if(m+k>v[mid]) lo=mid+1;
	}
	   return -1;
}

int main()
{int i,j;
    freopen("bete2.in","r",stdin);
    freopen("bete2.out","w",stdout);
	    scanf("%d",&x);
       for(i=1;i<=x;i++)
	       scanf("%d",&v[i]);
   aranjare();
       for(i=1;i<=x;i++)
	    for(j=i+1;j<x;j++)
	       {
		   a=v[i];b=v[j];caut(a,b);
	       }

     printf("%d",nr);
   return 0;

}