#include <cstdio>
#include <cmath>

struct Point
{
    double x, y;
};

int main(void)
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int nV;
    scanf("%d", &nV);
    Point *ptrP(new Point[nV + 1]);
    for(int i(0); i < nV; i++)
        scanf("%lf %lf", &ptrP[i].x, &ptrP[i].y);
    double dAns(0.0);
    ptrP[nV] = ptrP[0];
    for(int i(0); i < nV; i++)
        dAns += (ptrP[i].x * ptrP[i + 1].y - ptrP[i + 1].x * ptrP[i].y);
    printf ("%lf", fabs (dAns / 2.0));
    return 0;
}