#include <fstream>
#include<iomanip>
using namespace std;
#define Nmax 100005

ifstream fin("aria.in");
ofstream fout("aria.out");
struct punct{
    double x,y;
}a[Nmax];
double aria;
int n;

int main(){

    fin>>n;
    for(int i=1;i<=n;i++)
        fin>>a[i].x>>a[i].y;

    a[n+1]=a[1];

    for(int i=1;i<=n;i++)
        aria+=(a[i].x*a[i+1].y-a[i].y*a[i+1].x);


    aria/=2;
    fout<<fixed<<setprecision(6)<<aria;
    return 0;
}