#include <cstdio>
#define N 100002

using namespace std;
FILE *f,*g;

struct bla
{
    double x,y;
}v[N];
double  det(bla A, bla B,bla C)
{
    double x1=A.x,x2=B.x,x3=C.x,y1=A.y,y2=B.y,y3=C.y;
    return (x1*y2+x2*y3+x3*y1-x3*y2-y3*x1-x2*y1);
}
int main()
{
    f=fopen("aria.in","r");
    g=fopen("aria.out","w");
    int n;
    fscanf(f,"%d",&n);
    for(int i=1;i<=n;++i)
        fscanf(f,"%lf %lf",&v[i].x,&v[i].y);
    double A=0;
    for(int i=2;i<n;++i)
        A=A+det(v[1],v[i],v[i+1]);
    A=A*0.5;
    fprintf(g,"%f",A);

    fclose(f);
    fclose(g);
    return 0;
}