#include<fstream>
#include<iomanip>
using namespace std;

double abs(double x)
{
    if (x<0) return x*(-1);
    return x;
}

struct point
{
    double x,y;
};
point a[100007];
int i,n;
double sol;

int main()
{
    ifstream cin("aria.in");
    ofstream cout("aria.out");
    cin>>n;
    for (i=1; i<=n; ++i) cin>>a[i].x>>a[i].y;
    a[n+1].x=a[1].x;
    a[n+1].y=a[1].y;
    for (i=1; i<=n; ++i) sol+=a[i].x*a[i+1].y-a[i+1].x*a[i].y;
    sol/=2;
    cout<<setprecision(10)<<abs(sol);
    return 0;
}