#include<stdio.h>

int viz[7000],v[9001],i,j,k,l,m,n,sol;

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]);
        viz[v[i]]++;
        }

for(i=1;i<=n;i++)
        for(j=1;j<=n;j++)
           if(j!=i)
                {k=v[i]+v[j];
                if(k%2==0){
                k=k>>1;
                sol+=viz[k];
                if(k==v[i])sol--;
                if(k==v[j])sol--;}
                }

printf("%d",(sol/2));
return 0;}