#include <bits/stdc++.h>

using namespace std;

#ifdef DEBUG_MODE
#define DBG(n) n;
#else
#define DBG(n) ;
#endif
#define REP(i,n) for(ll (i) = (0);(i) < (n);++i)
#define rep(i,s,g) for(ll (i) = (s);(i) < (g);++i)
#define rrep(i,s,g) for(ll (i) = (s);i >= (g);--(i))
#define PB push_back
#define MP make_pair
#define FI first
#define SE second
#define SHOW1d(v,n) {for(int WWW = 0;WWW < (n);WWW++)cerr << v[WWW] << ' ';cerr << endl << endl;}
#define SHOW2d(v,i,j) {for(int aaa = 0;aaa < i;aaa++){for(int bbb = 0;bbb < j;bbb++)cerr << v[aaa][bbb] << ' ';cerr << endl;}cerr << endl;}
#define ALL(v) v.begin(),v.end()
#define Decimal fixed<<setprecision(20)
#define INF 1000000000
#define LLINF 1000000000000000000LL
#define MOD 1000000007

typedef long long ll;
typedef pair<ll, ll> P;

const double EPS = 1e-12;
const double PI = acos(-1.0);
typedef complex<double> point;
namespace std {
	bool operator < (const point& a, const point& b) {
		return real(a) != real(b) ? real(a) < real(b) : imag(a) < imag(b);
	}
	
	bool operator == (const point& a,const point& b) {
		return (abs(a.real() - b.real()) < EPS && abs(a.imag() - b.imag()) < EPS);
	}
}
struct circle {
	point p; double r;
	circle(){}
	circle(const point &p, double r) : p(p), r(r) { }
};
struct segment : public vector<point> {
	segment(const point &a, const point &b) {
		push_back(a); push_back(b);
	}
};

double cross(const point& a, const point& b) {
	return imag(conj(a)*b);
}

double dot(const point& a, const point& b) {
	return real(conj(a)*b);
}

/*
a → b で時計方向に折れて b → c
a → b で半時計方向に折れて b → c
a → b で逆を向いて a を通り越して b → c
a → b でそのまま b → c
a → b で逆を向いて b → c ( または b == c )
*/

int ccw(point a, point b, point c) {
	b -= a; c -= a;
	if (cross(b, c) > 0)   return +1;       // counter clockwise
	if (cross(b, c) < 0)   return -1;       // clockwise
	if (dot(b, c) < 0)     return +2;       // c--a--b on line
	if (norm(b) < norm(c)) return -2;       // a--b--c on line
	return 0;
}

bool intersectLL(const segment &l, const segment &m) {
	return abs(cross(l[1] - l[0], m[1] - m[0])) > EPS || // non-parallel
		abs(cross(l[1] - l[0], m[0] - l[0])) < EPS;   // same line
}
bool intersectLS(const segment &l, const segment &s) {
	return cross(l[1] - l[0], s[0] - l[0])*       // s[0] is left of l
		cross(l[1] - l[0], s[1] - l[0]) < EPS; // s[1] is right of l
}
bool intersectLP(const segment &l, const point &p) {
	return abs(cross(l[1] - p, l[0] - p)) < EPS;
}
bool intersectSP(const segment &s, const point &p) {
	return abs(s[0] - p) + abs(s[1] - p) - abs(s[1] - s[0]) < EPS; // triangle inequality
}
//端点の交差も考える
bool intersectSS(const segment &s, const segment &t) {
	if(intersectSP(s,t[0]) || intersectSP(s,t[1]) || intersectSP(t,s[0]) || intersectSP(t,s[1]))return true;
	return ccw(s[0], s[1], t[0])*ccw(s[0], s[1], t[1]) <= 0 &&
		ccw(t[0], t[1], s[0])*ccw(t[0], t[1], s[1]) <= 0;
}

point projection(const segment &l, const point &p) {
	double t = dot(p - l[0], l[0] - l[1]) / norm(l[0] - l[1]);
	return l[0] + t*(l[0] - l[1]);
}
point reflection(const segment &l, const point &p) {
	return p + 2. * (projection(l, p) - p);
}
double distanceLP(const segment &l, const point &p) {
	return abs(p - projection(l, p));
}
double distanceLL(const segment &l, const segment &m) {
	return intersectLL(l, m) ? 0 : distanceLP(l, m[0]);
}
double distanceLS(const segment &l, const segment &s) {
	if (intersectLS(l, s)) return 0;
	return min(distanceLP(l, s[0]), distanceLP(l, s[1]));
}
double distanceSP(const segment &s, const point &p) {
	const point r = projection(s, p);
	if (intersectSP(s, r)) return abs(r - p);
	return min(abs(s[0] - p), abs(s[1] - p));
}
double distanceSS(const segment &s, const segment &t) {
	if (intersectSS(s, t)) return 0;
	return min(min(distanceSP(s, t[0]), distanceSP(s, t[1])),
		min(distanceSP(t, s[0]), distanceSP(t, s[1])));
}
double distancePP(const point &a,const point &b){
	return abs(a-b);
}

//交点
point crosspoint(const segment &l, const segment &m) {
	if(!intersectSS(l,m))return point(INF,INF);
	double A = cross(l[1] - l[0], m[1] - m[0]);
	double B = cross(l[1] - l[0], l[1] - m[0]);
	if (abs(A) < EPS && abs(B) < EPS) return point(INF,INF); // same line
	if (abs(A) < EPS) return point(INF,INF); // !!!PRECONDITION NOT SATISFIED!!!
	return m[0] + B / A * (m[1] - m[0]);
}

//凸包
vector<point> convex_hull(vector<point> ps) {
	int n = ps.size(), k = 0;
	sort(ps.begin(), ps.end());
	vector<point> ch(2 * n);
	for (int i = 0; i < n; ch[k++] = ps[i++]) // lower-hull
		while (k >= 2 && ccw(ch[k - 2], ch[k - 1], ps[i]) <= 0) --k;
	for (int i = n - 2, t = k + 1; i >= 0; ch[k++] = ps[i--]) // upper-hull
		while (k >= t && ccw(ch[k - 2], ch[k - 1], ps[i]) <= 0) --k;
	ch.resize(k - 1);
	return ch;
}

/*多角形内包判定
OUT:0
ON:1
IN:2
*/
#define curr(PP, i) PP[i]
#define next(PP, i) PP[(i+1)%PP.size()]
int contains(const vector<point>& Poly, const point& p) {
	bool in = false;
	for (int i = 0; i < Poly.size(); ++i) {
		point a = curr(Poly, i) - p, b = next(Poly, i) - p;
		if (imag(a) > imag(b)) swap(a, b);
		if (imag(a) <= 0 && 0 < imag(b))
			if (cross(a, b) < 0) in = !in;
		if (cross(a, b) == 0 && dot(a, b) <= 0) return 1;
	}
	return in ? 2 : 0;
}

//見えるか(可視グラフ用) 
bool block_off(const point &a, const point &b, const vector<point> &obj) {
  point m = (a+b)/2.0;
  bool on = false, in = false;
  for (int j = 0; j < obj.size(); ++j) {
    point c = curr(obj,j), d = next(obj,j);
    if (imag(d) < imag(c)) swap(c, d);
    if (cross(a-c,b-c) * cross(a-d,b-d) < 0 &&    // strictly intersect.
        cross(c-a,d-a) * cross(c-b,d-b) < 0) return true;
    if (cross(a-c,b-c) == 0 && dot(a-c,b-c) < 0) return true;
    if (imag(c) <= imag(m) && imag(m) < imag(d))  // strictly contain.
      if (cross(c-m,d-m) < 0) in = !in;
    if (cross(c-m,d-m) == 0 && dot(c-m,d-m) <= 0) on = true;
  }
  return !on && in;
}

//面積
double area(const vector<point>& p) {
	double A = 0;
	for (int i = 0; i < p.size(); ++i)
		A += cross(curr(p, i), next(p, i));
	return A / 2.;
}

//角度足し算
double add_rad(double a,double b){
	double ret = a + b;
	if(ret > PI)ret -= 2 * PI;
	return ret;
}

//なす角(vector)
double angle(const point &a,const point &b) {
	auto tmp = abs(arg(a) - arg(b));
	return min(tmp, 2 * PI - tmp);
}

double angle(const segment &s1,const segment &s2) {
	return angle(s1[1] - s1[0], s2[1] - s2[0]);
}

//点の回転
point rotate(const point &p, double rad) {
	double X = p.real() * cos(rad) - p.imag() * sin(rad);
	double Y = p.imag() * cos(rad) + p.real() * sin(rad);
	return point(X, Y);
}

//並行
bool isParallel(const point &a, const point &b){
    return abs(cross(a,b)) < EPS;
}
bool isParallel(const segment &a, const segment &b){
    return isParallel(a[1]-a[0], b[1]-b[0]);
}
//直行
bool isOrthogonal(const point &a,const point &b){
	return abs(angle(a,b) - PI / 2) < EPS;
}
bool isOrthogonal(const segment &a,const segment &b){
	return isOrthogonal(a[1]-a[0],b[1]-b[0]);
}
int main() {
	
	int n;cin >> n;
	
	vector<point> poly;
	REP(i,n){
		double a,b;cin >> a >> b;
		poly.PB(point(a,b));
	}
	
	printf("%.1f\n",area(poly));

	return 0;
}



Measured memory_bytes: 3208