Polygon area

#include <bits/stdc++.h>

using namespace std;
using ll = long long;
typedef long long C;
typedef complex<C> P;
#define X real()
#define Y imag()

// Debug printing
#ifdef DEBUG
#define deb(fmt, args...) printf("DEBUG: %d: " fmt, __LINE__, ##args)
#define debug_print(fmt, args...) printf(fmt, ##args)
#else
#define deb(fmt, args...)
#define debug_print(fmt, args...)
#endif

void print_array(vector<int> in, const string title = "Vector")
{
	debug_print("DEBUG: %s [\nDEBUG: ", title.c_str());
	for (unsigned int i = 0; i < in.size(); i++) {
		debug_print("%d ", in[i]);
	}
	debug_print("\nDEBUG: ] END\n");
}

void print_matrix(vector<vector<int>> in, const string title = "Matrix")
{
	debug_print("DEBUG: %s [\nDEBUG: ", title.c_str());
	for (unsigned int i = 0; i < in.size(); i++) {
		for (unsigned int j = 0; j < in[i].size(); j++) {
			debug_print("%d ", in[i][j]);
		}
		debug_print("\nDEBUG: ");
	}
	debug_print("DEBUG: ] END\n");
}

int main(int argc, char *argv[])
{
	ios::sync_with_stdio(0);
	cin.tie(0);

	// Read the input parameters
	int n;
	cin >> n;

	// Read pairs from multiple lines
	vector<P> in(n + 1);
	int x, y;
	for (int i = 0; i < n; i++) {
		cin >> x >> y;
		in[i] = {x, y};
	}
	in[n] = in[0];

	ll sum = 0;
	for (int i = 0; i < n; i++) {
		P a = in[i];
		P b = in[i + 1];
		// deb("i: %d, a: (%d, %d), b: (%d, %d)\n", i, a.X, a.Y, b.X, b.Y);
		// ll cross = (conj(a)*b).Y;
		sum += (conj(a)*b).Y;
	}
	sum = abs(sum);
	cout << sum << "\n";
	
	return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED