/*
	http://www.infoarena.ro/problema/aria
*/
#pragma warning(disable:4996)
#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;

struct point { long double x, y; };

point a[100001];
int N;
double solutia;

void read()
{
	freopen("aria.in", "r", stdin);
	scanf("%d", &N);
	for (int i = 0; i < N; ++i)
	{
		scanf("%lf %lf", &a[i].x, &a[i].y);
	}
	a[N] = a[0];
}

void solve()
{
	for (int i = 0; i < N; ++i)
	{
		solutia += (a[i].x * a[i + 1].y - a[i + 1].x * a[i].y);
	}
	solutia /= 2;
}

void write()
{
	freopen("aria.out", "w", stdout);
	printf("%.6lf\n", solutia);
}

int main()
{
	read();
	solve();
	write();
	return 0;
}