#define _CRT_SECURE_NO_WARNINGS
#define _SILENCE_ALL_CXX17_DEPRECATION_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream>
#include <random>
#include <map>
#include <fstream>
#include <iomanip>
#include <time.h>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <assert.h>
#include <complex>
using namespace std;
#define LONG_INF 10000000000000
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007
#define MOD 998244353LL
#define seg_size 65536*4
#define REP(i,n) for(long long i = 0;i < n;++i)
class Point{
public:
	long double x;
	long double y;
	const Point operator*(const long double &a) {
		Point ans;
		ans.x = x * a;
		ans.y = y * a;
		return ans;
	}
	const Point operator=(const Point &a){
		x = a.x;
		y = a.y;
		return *this;
	}
	const Point operator+(const Point &a) const{
		Point ans;
		ans.x = a.x + x;
		ans.y = a.y + y;
		return ans;
	}
	const Point operator+=(const Point &a){
		x += a.x;
		y += a.y;
		return *this;
	}
	const Point operator-(const Point &a) const{
		Point ans;
		ans.x = x - a.x;
		ans.y = y - a.y;
		return ans;
	}
	const Point operator-=(const Point &a) {
		x -= a.x;
		y -= a.y;
		return *this;
	}
	static long double inner(const Point &a, const Point &b) {
		return a.x * b.x + a.y * b.y;
	}
	static long double cross(const Point &a, const Point &b) {
		return a.x * b.y - a.y * b.x;
	}
	static Point norms(const Point &a) {
		long double hoge = abs(a);
		Point ans;
		ans.x = a.x / hoge;
		ans.y = a.y / hoge;
		return ans;
	}
	static long double abs(const Point &a) {
		return sqrtl(a.x * a.x + a.y * a.y);
	}
};
int main(){
	int n;
	cin >> n;
	vector<Point> input(n + 1);
	REP(i, n) {
		cin >> input[i].x >> input[i].y;
	}
	input[n] = input[0];
	long double ans = 0;
	for (int i = 0; i < n; ++i) {
		ans += Point::cross(input[i], input[i + 1]);
	}
	ans = abs(ans);
	ans /= 2.0;
	cout << fixed << setprecision(1);
	cout << ans << endl;
	return 0;
}
