import java.util.Scanner;

public class C_GreedyMtSaka {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] X = new int[N];
        int[] Y = new int[N];
        for (int i = 0; i < N; i++) {
            X[i] = sc.nextInt();
            Y[i] = sc.nextInt();
        }
        System.out.println(-polygonAreaTwice(X, Y, N));
    }

    static long polygonAreaTwice(int[] X, int[] Y, int n) {
        long area = 0L;

        for (int i = 0, j = n - 1; i < n; j = i, i++) {
            area += (long) (X[j] + X[i]) * (Y[j] - Y[i]);
        }
        return area;
    }
}