#include<cstdio>
using namespace std;
int n,i;
double s,x,y,x1,x2,y1,y2;

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    scanf("%lf %lf",&x1,&y1);
    x=x1,y=y1;

    for(i=1;i<n;i++)
    {
        scanf("%lf %lf",&x2,&y2);
        s+=(x1*y2-x2*y1)/2;
        x1=x2,y1=y2;
    }
    s+=(x2*y-x*y2)/2;

    printf("%lf\n",s);
    return 0;
}