#include <fstream>
#include <cstdio>
#include <iomanip>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

struct area{
    double x, y;
} a[100005];

int n, i;
double s;

int main()
{
    fin >> n;
    for(i=1; i<=n; i++)
        fin >> 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;
    fout << setprecision(6) << fixed << s;
    return 0;
}