#include <stdio.h>
#include <math.h>

struct point{
    double x,y;
};
double det(point a,point b){
    return a.x*b.y-b.x*a.y;
}
int main(){
    freopen("aria.in","r",stdin);
    unsigned long n;
    point a,b,c;
    scanf("%lu",&n);
    scanf("%lf %lf",&a.x,&a.y);
    c=a;
    double surface=0;
    for(unsigned long i=1;i<n;i++){
        scanf("%lf %lf",&b.x,&b.y);
        surface+=det(a,b);
        a=b;
    }
    fclose(stdin);
    surface+=det(a,c);
    freopen("aria.out","w",stdout);
    printf("%.5lf",fabs(surface/2.0));
    fclose(stdout);
    return 0;
}