#include <fstream>
#define MAXN 100005
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

int n;
long double x[MAXN],y[MAXN],a;

int main()
{
    int i;
    f>>n;
    f>>x[1]>>y[1];
    for(i=2;i<=n;i++){
        f>>x[i]>>y[i];
        a+=((y[i]+y[i-1])*(x[i]-x[i-1])/2);}
    a+=(y[1]+y[n])*(x[1]-x[n])/2;
    g<<((a>0)?a:(-a));
    f.close();
    g.close();
    return 0;
}