#include <bits/stdc++.h>

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

int N;
double X[100001],Y[100001];

int main(){
	fin >>N;
	double rs=0;
	for (int i=1;i<=N;i++) fin >>X[i]>>Y[i];
	X[0]=X[N];
	Y[0]=Y[N];
	X[N+1]=X[1];
	Y[N+1]=Y[1];
	for (int i=1;i<=N;i++){
		rs+=X[i]*(Y[i+1]-Y[i-1]);
		
	}
	fout <<fixed<<setprecision(5)<<rs/2.0;
	return 0;
}