#include <iostream>
#include <iomanip>
#include <fstream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <vector>
#include <stack>
#include <algorithm>
using namespace std;
#define MAX 100010

ifstream f("aria.in");
ofstream g("aria.out");

struct Punct {
	long double x, y;
	Punct() {
		x = 0, y = 0;
	}
	Punct(long double x, long double y) {
		this->x = x, this->y = y;
	}	
} O, P[MAX];

long double DET(Punct p1, Punct p2, Punct p3) {
	return p1.x*p2.y + p2.x*p3.y + p3.x*p1.y - p2.y*p3.x - p3.y*p1.x - p1.y*p2.x;
}

int N;
long double A;

int main() {
	
	f >> N;
	for (int i = 0; i < N; i++) {
		f >> P[i].x >> P[i].y;
	}
	for (int i = 0; i < N; i++) {
		A += 0.5 * DET(O, P[i], P[(i+1) % N]);
	}
	g << setprecision(10) << A;
	
	return 0;
}