#include <cstdio>
using namespace std;
struct pct
{
    double x,y;
}x[100002];
int n;
double sum;
void citire()
{
    freopen("aria.in","r",stdin);
    scanf("%d",&n);
    for(int i=1;i<=n;++i)
        scanf("%lf%lf",&x[i].x,&x[i].y);
    fclose(stdin);
    x[n+1]=x[1];
}
void solve()
{
    for(int i=1;i<=n;++i)
        sum+=(x[i].x*x[i+1].y-x[i+1].x*x[i].y);
}
void afisare()
{
    freopen("aria.out","w",stdout);
    printf("%.5lf",sum/2);
    fclose(stdout);
}
int main()
{
    citire();
    solve();
    afisare();
    return 0;
}