#include<cstdio>

using namespace std;

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

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]);
    }
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(i=1;i<=n;i++)
    {
        ans+=x[i]*y[i+1]-y[i]*x[i+1];
    }
    printf("%.5lf\n",ans/2);
}