#include <cstdio>
#include <cmath>
#include <algorithm>
#define x first
#define y second
using namespace std;

const int NMAX = 100003;
pair <double, double> P[NMAX];
double S;

int main () {

    freopen ("aria.in", "r", stdin);
    freopen ("aria.out", "w", stdout);
    int N, i;
    scanf ("%d", &N);
    P[N] = P[0];
    for (i = 0; i < N; ++i)
        scanf ("%lf%lf", &P[i].x, &P[i].y);
    P[N] = P[0];
    for (i = 0; i <= N; ++i)
        S += P[i].x * P[i + 1].y - P[i + 1].x * P[i].y;
    printf ("%.6lf", fabs (S / 2));

}