#include <cstdio>
#define nmax 100001
using namespace std;
FILE *f1=fopen("arie.in","r"),*f2=fopen("arie.out","w");
int n,i;
double A;
struct point
{
    double x,y;
}v[nmax];
int main()
{

    fscanf(f1,"%d",&n);
    for(i=1;i<=n;i++)
        fscanf(f1,"%lf%lf",&v[i].x,&v[i].y);
    v[n+1].x=v[1].x;v[n+1].y=v[1].y;
    for(i=0;i<=n+1;i++)
        A+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    fprintf(f2,"%lf",A/2.0);

    fclose(f1);
    fclose(f2);
    return 0;
}