#include <cstdio>
#include <cmath>
using namespace std;

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

	int n;
	scanf("%d", &n);

	double aria = 0;
	double x[n+2], y[n+2];
	for(int i=0; i<n; ++i)
		scanf("%lf%lf", &x[i], &y[i]);

	x[n] = x[0];
	y[n] = y[0];

	for(int i=0; i<n; ++i)
		aria += (x[i] * y[i+1] - x[i+1] * y[i]);

    printf("%f\n", fabs(aria / 2.0));

	return 0;
}