#include <cstdio>

using namespace std;

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

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

    int i;

	scanf("%d", &N);

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

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

    printf("%lf", arie / 2);
	fclose(stdout);
    return 0;
}