#include <stdio.h>
#include <iomanip>
using namespace std;
FILE *f=fopen("aria.in","r");
FILE *g=fopen("aria.out","w");
long n,i,j;
double s;
struct ab{
    double x,y;
};
ab v[100001];
int main()
{
    fscanf(f,"%ld",&n);
    for (i=1;i<=n;i++)
        fscanf(f,"%lf%lf",&v[i].x,&v[i].y);
    for (i=1;i<=n-1;i++)
        s+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y)/2;
    s+=(v[n].x*v[1].y-v[1].x*v[n].y)/2;
    //g.precision(10);
    fprintf(g,"%lf",s);
    fclose(f);
    return 0;
}