#include <bits/stdc++.h>

using namespace std;

typedef pair<double,double> P;
 
int main(){
  int N;
 
  cin >> N;
  vector<P> v;
  double ans = 0, x ,y;
  for(int i = 0 ; i < N ; i++){
    cin >> x >> y;
    v.push_back(P(x,y));
  } 
  for(int i = 1; i < N+1 ; i++){
    ans += (v[i%N].first-v[i-1].first)*(v[i%N].second+v[i-1].second);
  }
  ans /= 2.0;
  printf("%.1f\n" ,abs(ans));
  
  return 0;
}