#include <stdio.h>

using namespace std;

FILE*f=fopen("medie.in","r");
FILE*g=fopen("medie.out","w");

#define ll long long

ll m[7001],v[7001];

int main()
{
    ll n,nr=0,i,j,x;
    fscanf(f,"%lld",&n);
    for (i=1;i<=n;i++) {
        fscanf(f,"%lld",&v[i]);
        m[v[i]]++;
    }
    for (i=1;i<n;i++)
        for (j=i+1;j<=n;j++)
            if ((v[i]+v[j])%2==0){
                x=(v[i]+v[j])/2;
                nr+=m[x];
                if (v[i]==x) nr-=2;
            }
    fprintf(g,"%lld",nr);
    fclose(f);
    fclose(g);
    return 0;
}