#include<iostream>
#include<fstream>
using namespace std;

int n;
ifstream fi("aria.in");
ofstream fo("aria.out");
int xs[100005];
int ys[100005];

int main() {
	double aria = 0;
	fi >> n;
	for (int i = 0; i < n; i++) {
		fi >> xs[i] >> ys[i];
	}
	for (int i = 0; i < n - 1; i++) {
		aria += (xs[i] * ys[i + 1] - xs[i + 1] * ys[i]);
	}
	aria += xs[n - 1] * ys[0] - xs[0] * ys[n - 1];
	aria *= 0.5;
	fo << aria;
	fi.close();
	fo.close();
	return 0;	
}