#include <stdio.h>

int n,i,j,a[9001],c;
int v[14002],f[7001];

int main(){
    freopen("medie.in","r",stdin);
    freopen("medie.out","w",stdout);
    
    scanf("%ld",&n);
    for (i=1;i<=n;i++)
        scanf("%ld",&a[i]);
    for (i=1;i<n;i++){
        f[a[i]]++;
        for (j=i+1;j<=n;j++)if((a[i]+a[j])%2==0)
            v[(a[i]+a[j])/2]++;
    }
    f[a[n]]++;
    for (i=1;i<=n;i++)if(v[a[i]])c+=v[a[i]]-f[a[i]]+1;
    
    printf("%ld\n",c);
    
    return 0;
}