#include<cstdio>
struct punct
{
    double x,y;
}v[100013];
int n,i;
double a;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for(i=0;i<n;++i)scanf("%lf%lf",&v[i].x,&v[i].y);
    for(i=1;i<n;++i) a+=v[i].x*v[i+1].y-v[i].y*v[i+1].x;
    if(a<0)a=-a;
    printf("%.4lf",a);
    return 0;
}