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

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

typedef long double ld;
typedef pair<ld, ld> pd;

const int NMAX=1e5;

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

int main() {
	ld A=0.0;
	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+=P[i].first*P[i+1].second-P[i].second*P[i+1].first;
	}
	A/=2.0;
	if(A<0)
		A=-A;
	fout<<fixed<<setprecision(6)<<A<<'\n';
	return 0;
}