#include<cstdio>
using namespace std;
int v[9010],vc[7010];
int main(){
    freopen("medie.in","r",stdin);
    freopen("medie.out","w",stdout);
    int n,i,j,c,b;
    long long sol=0;
    scanf("%d",&n);
    for(i=1;i<=n;i++){
        scanf("%d",&v[i]);
        vc[v[i]]++;
    }
    for(i=1;i<n;i++)
        for(j=i+1;j<=n;j++)
            if((v[i]+v[j])%2==0){
                sol+=vc[(v[i]+v[j])/2];
                if(v[i]==v[j])
                    sol-=2;
            }
    printf("%lld",sol);
    return 0;
}