#include<iostream>
#include<cstdio>
#include<complex>
#include<cmath>

using namespace std;

typedef complex<double> xy_t;
double cross(xy_t a,xy_t b);
xy_t P[110];



int main(){
  
  int N;

  double x,y;
  
  cin >> N;
  
  for(int i=0;i<N;i++){
    cin >> x >> y;
    P[i] = xy_t(x,y);
  
  }
  
 
  double ans=0;
  for(int j=1;j<N-1;j++){
    ans+=cross(P[j]-P[0],P[j+1]-P[0]);
  }

  ans=abs(ans)/2;
  cout.precision(1);
  cout << fixed << ans << endl;
  
}
double cross(xy_t a, xy_t b){return (conj(a)*b).imag();}