# include <cassert>
	
# include <cstdio>
	
# include <cmath>
	
 
	
const char *FIN = "aria.in", *FOU = "aria.out";
	
const int MAX = 100005;
	
 
	
struct punct {
	
    double x, y;
	
} vec[MAX];
	
 
	
int N;
	
double sol;
	
 
	
int main (void) {
	
    assert (freopen (FIN, "r", stdin));
	
    assert (freopen (FOU, "w", stdout));
	
 
	
    assert (scanf ("%d", &N) == 1);
	
    assert (1 <= N && N <= 100000);
	
    for (int i = 0; i < N; ++i) {
	
        assert (scanf ("%lf %lf", &vec[i].x, &vec[i].y) == 2);
	
        assert (-10000000 <= vec[i].x && vec[i].x <= 10000000);
	
        assert (-10000000 <= vec[i].y && vec[i].y <= 10000000);
	
    }
	
    vec[N] = vec[0];
	
    for (int i = 0; i < N; ++i)
	
        sol += (vec[i].x * vec[i + 1].y - vec[i + 1].x * vec[i].y);
	
    printf ("%lf", fabs (sol / 2.0));
	
}