#include <bits/stdc++.h>
//#include "test.h"
using namespace std;

vector<pair<int,int>> v(100002);

//pair<int,int> v[100002];

int inm(int a, int b);

int main(){
    //cout<<inm(2,3)<<' '<<suma_si_inm(2,5);
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    int n, x, y;
    fin>>n;
    for(int i=1; i<=n; i++)
    	fin>>v[i].first>>v[i].second;
  	//facem sa fie sir circular, adica dupa ult elem pune primul
  	v[n+1].first = v[1].first;
  	v[n+1].second = v[1].second;

  	double aria=0.0; //aria la poligon
    for(int i=1; i<=n; i++) //aici facem ariile la fiecare triunghi ce compun poligonul
    {
		aria+=v[i].first*v[i+1].second - v[i+1].first*v[i].second;
    }
  	aria*=1.0/2.0; //1/2 cum ambele sunt int da 0, dar daca faci 1.0/2.0 rezulta 0.5 ca sunt double(float)
  	fout<<aria;
}

int inm(int a, int b){
    return a*b;
}