#include<fstream>
#include<cmath>
#define NMAX 100005
#define x first
#define y second
using namespace std;
ifstream eu("aria.in");
ofstream tu("aria.out");
int n;
double aria=0;
pair<double,double> v[NMAX];
int main()
{
    int i;
    eu>>n;
    for(i=0;i<n;i++)
        eu>>v[i].x>>v[i].y;
    v[n++]=v[0];
    for(int i=0;i<n;i++)
        aria=aria+(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    aria=aria/2;
    tu<<fixed<<abs(aria)<<"\n";
    return 0;
}