#include <bits/stdc++.h>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct punct
{
    double x;
    double y;
};
punct p[100001];
int n, i;
double a;
int main()
{
    fin>>n;
    fin>>p[1].x>>p[1].y;
    for(i=2;i<=n;i++)
    {
        fin>>p[i].x>>p[i].y;
        a+=(p[i-1].x*p[i].y-p[i].x*p[i-1].y)/2;
    }
    a+=(p[n].x*p[1].y-p[1].x*p[n].y)/2;
    fout << fixed << setprecision(5) << a;
    return 0;
}