#include <cstdio>

using namespace std;

int v[9010],vaz[7010];

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