#include <iostream>
#include <fstream>
#include <iomanip>
#include <cmath>
using namespace std;
struct aria{double x, y;};
aria a[100005];
int i, n; double s;
int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");
    f>>n;
    for(i=1; i<=n; i++) f >> a[i].x >> a[i].y;
    a[n+1]=a[1];
    for(i=1; i<=n; i++) s+=(a[i].x*a[i+1].y-a[i+1].x*a[i].y);
    s/=2.0;
    if(s<0) s=s* -1;
    g<<setprecision(20)<<s;
    return 0;
}