Polygon area

#include <bits/stdc++.h>
using namespace std;

#define I_2D(row, col, width) ((row) * (width) + (col))
#define PRINT_ARR(arr, n)                                                      \
  do {                                                                         \
    for (int i = 0; i < n; i++) {                                              \
      cout << arr[i] << " ";                                                   \
    }                                                                          \
    cout << "\n";                                                              \
  } while (0)
#define PRINT_VEC_ARR(v, n)                                                    \
  do {                                                                         \
    for (int i = 0; i < n; i++) {                                              \
      cout << i << ": ";                                                       \
      for (auto item : v[i]) {                                                 \
        cout << item << " ";                                                   \
      }                                                                        \
      cout << endl;                                                            \
    }                                                                          \
  } while (0)
#define endl '\n';

typedef long long ll;
typedef complex<ll> P;
#define X real()
#define Y imag()
#define CP(a, b) ((conj(a) * b).Y)

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  P arr[n];
  ll x, y;
  for (int i = 0; i < n; i++) {
    cin >> x >> y;
    arr[i] = {x, y};
  }

  ll sum = 0;
  for (int i = 0; i < n; i++) {
    sum += (arr[i].X * arr[(i+1) % n].Y - arr[i].Y * arr[(i+1) % n].X);
  }
  cout << abs(sum) << endl;
  return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED