#include <iostream>
#include <vector>
#include <cmath>
using namespace std;
int main() {
    int N;
    cin >> N;

    vector<Point> points(N);
    for (int i = 0; i < N; i++) {
        cin >> points[i].x >> points[i].y;
    }

    int result = area(points);
    cout << result << endl;

    return 0;
}