#include <cmath>
#include <cstdio>
#define nmax 100002
using namespace std;
FILE *f=fopen("aria.in","r"),*g=fopen("aria.out","w");
struct punct{
    double x,y;
}v[nmax];
double arie;
double det(punct a, punct b)
{
    return a.x*(b.y-1)-b.x*(a.y-1)+a.y-b.y;
}
int main()
{
    int i,j,n;
    fscanf(f,"%d",&n);
    for(i=1;i<=n;i++)
        fscanf(f,"%lf %lf",&v[i].x,&v[i].y);
    v[++n]=v[1];
    for(i=1;i<=n;i++)
        arie+=det(v[i],v[i+1]);
    fprintf(g,"%.6lf",abs(arie/2.0));
    fclose(f);
    fclose(g);
    return 0;
}