#include<cstdio>
using namespace std;
int n;
double s,x[100005],y[100005];
int main()
{
    int i;
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for (i=1;i<=n;++i)
        scanf("%lf%lf",&x[i],&y[i]);
    for (i=1;i<n;++i)
        s+=x[i]*y[i+1]-x[i+1]*y[i];
    s+=x[n]*y[1]-x[1]*y[n];
    printf("%.5lf\n",s/2);
    return 0;
}