#include<cstdio>

using namespace std;

double ans,x[100002],y[100002];
int i,n;

double arie(int ind1,int ind2,int ind3)
{
    return((x[ind1]*y[ind2]+x[ind2]*y[ind3]+x[ind3]*y[ind1]-x[ind1]*y[ind3]-x[ind2]*y[ind1]-x[ind3]*y[ind2])/2);
}

int main()
{
    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=2;i<n;i++)
    {
        ans+=arie(1,i,i+1);
    }
    printf("%lf\n",ans);
}