# include <cstdio>
# include <cmath>
 
const char *FIN = "aria.txt", *FOU = "aria.out";
const int MAX = 100005;
 
struct punct {
    double x, y;
} vec[MAX];
 
int N;
double sol;
 
int main (void) {
    freopen (FIN, "r", stdin);
    freopen (FOU, "w", stdout);
 
    scanf ("%d", &N);
    for (int i = 0; i < N; ++i)
        scanf ("%lf %lf", &vec[i].x, &vec[i].y);
    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));
}