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

ifstream fin("aria.in");
ofstream fout("aria.out");

typedef pair<double, double> pd;

const int NMAX=1e5;

int N;
pair<double,double> P[NMAX+2];

double trap(pd a, pd b) {
	double S=a.first*b.second-b.first*a.second;
	return S;
}

int main() {
	double A=0;
	fin>>N;
	for(int i=1;i<=N;++i) {
		fin>>P[i].first>>P[i].second;
	}
	P[N+1]=P[1];
	for(int i=1;i<=N;++i) {
		A+=trap(P[i], P[i+1]);
	}
	fout<<fixed<<setprecision(6)<<abs(A/2.0)<<'\n';
	return 0;
}