#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <algorithm>
#include <sstream>
#include <map>
#include <set>
#include <cmath>

#define REP(i,k,n) for(int i=k;i<n;i++)
#define rep(i,n) for(int i=0;i<n;i++)
#define INF 1<<30
#define pb push_back
#define mp make_pair
#define EPS 1e-8
#define equals(a,b) fabs((a) - (b)) < EPS

using namespace std;
typedef long long ll;
typedef pair<int,int> P;

struct Point {
	double x, y;

	Point(double x=0, double y=0) : x(x), y(y) {}

	Point operator+(const Point &o) const { return Point(x+o.x, y+o.y); }

	Point operator-(const Point &o) const { return Point(x-o.x, y-o.y); }

	Point operator*(const double m) const { return Point(x*m, y*m); }

	Point operator/(const double d) const { return Point(x/d, y/d); }

	bool operator<(const Point &o) const { return x != o.x ? x < o.x : y < o.y; }

	bool operator==(const Point &o) const { return fabs(x-o.x) < EPS && fabs(y-o.y) < EPS; }
};

ostream& operator << (ostream& os, const Point& p) {
	os << "(" << p.x << ", " << p.y << ")";
	return os;
}

double dot(Point a, Point b) { return a.x * b.x + a.y * b.y; }
double cross(Point a, Point b) { return a.x * b.y - a.y * b.x; }
double atan(Point p) { return atan2(p.y, p.x); }
double norm(Point p) { return p.x * p.x + p.y * p.y; }
double abs(Point p) { return sqrt(norm(p)); }
double distancePP(Point p, Point o) { return sqrt(norm(o - p)); }

double polygonArea(const vector<Point>& p) {
	int n = p.size();
	double ret = 0.0;
	for(int i = 0; i < n; ++i) {
		ret += cross(p[i], p[(i+1)%n]);
	}

	return abs(ret) / 2.0;
}

int ccw(Point a, Point b, Point c) {
	b = b-a;
	c = c-a;

	if(cross(b, c) > 0.0) return +1;	//conter clockwise
	if(cross(b, c) < 0.0) return -1;	//clockwise
	if(dot(b, c) < 0.0) return +2;	//a on Seg(b,c)
	if(norm(b) < norm(c)) return -2;	//b on Seg(a,c)
	return 0;	//c on Seg(a,b)
}

struct Line {
	Point a, b;

	Line() : a(Point(0, 0)), b(Point(0, 0)) {}

	Line(Point a, Point b) : a(a), b(b) {}
};

ostream& operator << (ostream& os, const Line& l) {
	os << "(" << l.a.x << ", " << l.a.y << ")-(" << l.b.x << "," << l.b.y << ")";
	return os;
}

struct Seg {
	Point a,b;

	Seg() : a(Point(0, 0)), b(Point(0, 0)) {}

	Seg (Point a, Point b) : a(a),b(b) {}
};

ostream& operator << (ostream& os, const Seg& s) {
	os << "(" << s.a.x << ", " << s.a.y << ")-(" << s.b.x << "," << s.b.y << ")";
	return os;
}

bool isOrthogonal(Line l1, Line l2) { return equals(dot((l1.b - l1.a), (l2.b - l2.a)), 0.0); }

bool isParallel(Line l1, Line l2) { return equals(cross((l1.b - l1.a), (l2.b - l2.a)), 0.0); }

bool sameLine(Line l1, Line l2) { return abs(cross(l1.b - l1.a, l2.b - l1.a)) < EPS; }

bool isIntersectLL(Line l1, Line l2) { return !isParallel(l1, l2) || sameLine(l1, l2); }

bool isIntersectLS(Line l, Seg s) {
	return cross(l.b - l.a, s.a - l.a) * cross(l.b - l.a, s.b - l.a) < 0;
}

bool isIntersectSS(Seg s1, Seg s2) { 
	return ccw(s1.a, s1.b, s2.a) * ccw(s1.a, s1.b, s2.b) <= 0 
		&& ccw(s2.a, s2.b, s1.a) * ccw(s2.a, s2.b, s1.b) <= 0;
}

double distanceLP(Line l, Point p) {
	return abs(cross(l.b - l.a, p - l.a)) / abs(l.b - l.a);
}

double distanceLS(Line l, Seg s) {
	if (isIntersectLS(l, s)) return 0.0;
	return min(distanceLP(l, s.a), distanceLP(l, s.b));
}

double distanceSP(Seg s, Point p) {
	if (dot(s.b - s.a, p - s.a) < 0.0) return abs(p - s.a);
	if (dot(s.a - s.b, p - s.b) < 0.0) return abs(p - s.b);
	return distanceLP(Line(s.a, s.b) , p);
}

double distanceSS(Seg s1, Seg s2) {
    if (isIntersectSS(s1, s2)) return 0.0;
	return min( min(distanceSP(s1, s2.a), distanceSP(s1, s2.b)), min(distanceSP(s2, s1.a), distanceSP(s2, s1.b)) );
}

// if isIntersectLL(l1, l2)
Point crossPointLL(Line l1, Line l2) {
	Point base = l2.b - l2.a;
	double d = abs(cross(base, l1.a - l2.a));
	double d2 = abs(cross(base, l1.b - l2.a));
	double t = d / (d + d2);
	return l1.a + (l1.b - l1.a) * t;
}

// if isIntersectLS(l, s)
Point crossPointLS(Line l, Seg s) { return crossPointLL(l, Line(s.a, s.b)); }

// if isIntersectSS(s1, s2)
Point crossPointSS(Seg s1, Seg s2) { return crossPointLL(Line(s1.a, s1.b), Line(s2.a, s2.b)); }

Point project(Line l, Point p) {
	Point base = l.b - l.a;
	double t = dot(base, p-l.a) / dot(base, base);
	return l.a + base * t;
}

Point reflect(Line l, Point p) {
	return p + (project(l, p) - p) * 2.0;
}

int main() {
	int n;
	cin >> n;

	vector<Point> v(n);
	rep(i, n) cin >> v[i].x >> v[i].y;

	cout << fixed;
	cout.precision(1);
	cout << polygonArea(v) << endl;

	return 0;
}

Measured memory_bytes: 1296