#include <cstdio>
#include <cmath>
using namespace std;
struct puncte_poligon{
    double x,y;
}p[100002];
int n;
double arie;
int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    scanf("%d", &n);
    for(int i=0;i<n;++i)
        scanf("%lf%lf", &p[i].x,&p[i].y);
    p[n]=p[0];

    for(int i=0;i<n;++i)
        arie+=(p[i].x*p[i+1].y-p[i+1].x*p[i].y);
    printf("%f",fabs(arie / 2.0));
    return 0;
}