#include <bits/stdc++.h>
#define int long long
const int N = 1e6 + 7;
using namespace std;
signed main()
{
  freopen ("ESUM.INP", "r", stdin);
  freopen ("ESUM.OUT", "w", stdout);
  int n, a = 0, b = 0, s = 0;
  vector <int> arr(N);
  cin >> n;
  for (int i = 1; i <= n; i ++)
  {
    cin >> arr[i];
    s += arr[i];
    if (arr[i] % 2 == 0) a++;
    else b++;
  }
  if (s % 2 == 0) cout << a * (a - 1) / 2 + b * (b - 1) / 2;
  else cout << a * b;
}

Chúc bn học tốt!

$\color{Turquoise}{\triangleright \triangleright\ \color{ Turquoise}{\text{ThienPhuc123} \triangleleft \triangleleft}}$

#include <bits/stdc++.h>
#define ll long long

using namespace std;

ll n, cnt1 = 0, cnt2 = 0, s = 0;
ll a[1000005];

int main()
{
    cin >> n;
    for (int i=1;i<=n;i++){
        cin >> a[i];
        s += a[i];
        (a[i]%2 == 0) ? cnt2++ : cnt1++;
    }
    if (s % 2 == 0){
        cout << cnt1*(cnt1-1)/2 + cnt2*(cnt2-1)/2;
    }
    else {
        cout << cnt1*cnt2;
    }
    return 0;
}