#include <cstdio>
using namespace std;
struct aa{double x,y;};
aa v[100002];
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    double aria=0;
    int n,i;
    scanf("%d",&n);
    for(i=1;i<=n;i++)
        scanf("%lf%lf",&v[i].x,&v[i].y);
    v[n+1]=v[1];
    for(i=1;i<=n;i++)
        aria+=((v[i].x*v[i+1].y)-(v[i].y*v[i+1].x))/2;
    printf("%.6f",aria);
    return 0;
}