#include <bits/stdc++.h>
#define x real()
#define y imag()
using namespace std;
typedef complex <double> point;
vector <point> po;
double dot(point a,point b){return a.x*b.x+a.y*b.y;}
double cross(point a,point b){return a.x*b.y-a.y*b.x;}

double get_area(){
  double res=0;
  int size=po.size();
  for(int i=0;i<size;i++){
    point a=po[i],b=po[(i+1)%size];
    res+=cross(a,b)/2;
  }
  return abs(res);
}


int main(){
  int n;
  cin>>n;
  for(int i=0,a,b;i<n;i++)cin>>a>>b,po.push_back(point(a,b));
  printf("%.1f\n",get_area());
  return 0;
}