#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
pair<long double,long double>v[100010];
int n;
long double aria;
int main() {

    fin >> n;
 	for(int i=1;i<=n;i++){
        fin>>v[i].first>>v[i].second;
 	}
 	for(int i=3;i<=n;i++){
        aria+=((v[i-1].first- v[1].first)*(v[i].second-v[1].second)-(v[i].first -v[1].first)*(v[i-1].second-v[1].second));
 	}
 	aria/=2;
 	if(aria<0){
        aria*=-1;
 	}
 	fout<<fixed<<setprecision(6)<<aria;





	return 0;
}