#include<cstdio>
using namespace std;
double x[100010],y[100010];
int main (){
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int n,i;
    double arie=0;
    scanf("%d",&n);
    for(i=1;i<=n;i++)
        scanf("%lf%lf",&x[i],&y[i]);
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(i=1;i<=n;i++)
        arie=arie+(x[i]*y[i+1]-x[i+1]*y[i]);
    printf("%.5f",arie/2);
    return 0;
}