#include<cstdio>
using namespace std;
const int NMAX = 100005;
int n,i; double A;
struct punct{double x,y;}P[NMAX];
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for(i=1;i<=n;i++) scanf("%lf%lf",&P[i].x,&P[i].y);
    P[n+1]=P[1];
    for(i=1;i<=n;i++) A+=(P[i].x*P[i+1].y-P[i+1].x*P[i].y);
    A/=2; printf("%lf\n",A);
    return 0;
}