#include<cstdio>
struct punct
{
    double x,y;
}a[10001];
punct p1,p2,po;
int i,n;
double s;
int main()
{
freopen("aria.in","rt",stdin);
freopen("aria.out","wt",stdout);
scanf("%ld",&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+=(a[i].y+a[i+1].y)*(a[i+1].x-a[i].x);
    if(s<0) s=-s;
    printf("%.5lf",s/2);
}