#include<fstream>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n,i;
double s,x[100004],y[100004];
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    f>>n;
    for(i=1;i<=n;++i) f>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(i=1;i<=n;++i) s+=x[i]*y[i+1]-x[i+1]*y[i];
    s=s/2.0;
    g<<s;
    return 0;
}