#include<cstdio>
using namespace std;
int n,i;
double s,x2,y2,x1,y1,x,y,d;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    scanf("%lf %lf",&x1,&y1);
    x2=x1;
    y2=y1;
    //x1 y1 1
    //x  y  1
    //0  0  1
    for(i=2;i<=n;++i)
    {
        scanf("%lf %lf",&x,&y);
        d=x1*y-x*y1;
        s+=d;
        x1=x;
        y1=y;
    }
    d=x1*y2-x2*y1;
    s+=d;
    s=(double)s/2;
    printf("%lf",s);
    return 0;
}