#include<cstdio>
#define LNG 110000
using namespace std;
struct point {double x,y;};
point a[LNG];
int n,i;
double S=0;
inline double modul(double x)
{
    if (x<0) return -x;
        else return x;
}
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for (i=1;i<=n;i++)
        scanf("%lf%lf",&a[i].x,&a[i].y);
    a[n+1]=a[1];
    for (i=1;i<=n;i++)
        S+=modul((double)(((double)((double)(a[i].x*a[i+1].y)-(double)(a[i+1].x*a[i].y)))/2));
    printf("%.10lf",S);
    fclose(stdin);
    fclose(stdout);
    return 0;
}