#include <cstdio>

using namespace std;

int N;
double x[100010], y[100010];
double arie;

int main()
{
    freopen("arie.in", "r", stdin);
	freopen("arie.out", "w", stdout);

    int i;

	scanf("%d", &N);

	for (i = 0; i < N; ++i)
        scanf("%lf %lf", &x[i], &y[i]);

    for (i = 0; i < N - i; ++i)
        arie += x[i] * y[i + 1] - x[i + 1] * y[i];

    if (arie < 0)
        arie = -arie;

    printf("%.5lf", arie);
	fclose(stdout);
    return 0;
}