#include <fstream>
#include <iomanip>
#define DIM 100010
#define x first
#define y second
using namespace std;

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

int n, m, i, j, k, ok, minim;
pair <double , double > v[DIM];
double sum;

void SetUp(){
     fin >> n;
     for(i = 1; i <= n; i ++)
          fin >> v[i].x >> v[i].y;
     v[n+1].x = v[1].x;
     v[n+1].y = v[1].y;
     return;
}

void Array(){
     sum = 0;
     for(i = 1; i <= n; i ++){
          sum += (v[i].x * v[i+1].y - v[i+1].x * v[i].y) / 2;
     }
     fout << setprecision(8) << fixed << sum;
     return;
}

int main(){
     SetUp();
     Array();
     return 0;
}