#include <cstdio>

using namespace std;

FILE *fin = fopen("medie.in", "r");
FILE *fout = fopen("medie.out", "w");

int n,i,j,v[9005],f[7005],rest[7005];

int main()
{
    fscanf(fin, "%d", &n);
    for (i=1; i<=n; i++)
    {
        fscanf(fin, "%d", &v[i]);
        f[v[i]]++; rest[v[i]] = v[i]%2;
    }
    long long sol = 0;
    for (i=1; i<n; ++i)
        for (j=i+1; j<=n; ++j)
            if (rest[v[i]] == rest[v[j]])
            {
                int val = (v[i]+v[j])/2;
                if (f[val])
                {
                    int ok = 0;
                    if (v[i] == val)
                        ok++;
                    if (v[j] == val)
                        ok++;
                    sol += f[val]-ok;
                }
            }
    fprintf(fout, "%lld", sol);
    return 0;
}