#define __USE_MINGW_ANSI_STDIO 0
#include <bits/stdc++.h>

using namespace std;
using ll = long long;
#define int ll
using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
#define PB push_back

const ll LLINF = (1LL<<60);
const int INF = (1LL<<30);
const int MOD = 1000000007;

template <typename T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template <typename T> T &chmax(T &a, const T &b) { return a = max(a, b); }
template <typename T> bool IN(T a, T b, T x) { return a<=x&&x<b; }
template<typename T> T ceil(T a, T b) { return a/b + !!(a%b); }
template<class S,class T>
ostream &operator <<(ostream& out,const pair<S,T>& a){
  out<<'('<<a.first<<','<<a.second<<')';
  return out;
}
template<class T>
ostream &operator <<(ostream& out,const vector<T>& a){
  out<<'[';
  REP(i, a.size()) {out<<a[i];if(i!=a.size()-1)out<<',';}
  out<<']';
  return out;
}

int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};

const double EPS = 1e-8;

using R = long double;
using P = complex<R>;
using L = pair<P,P>;
using G = vector<P>;
using C = pair<P,R>;
struct S : public L {
  S() {}
  S(const P &a, const P &b) : L(a,b) {}
};

inline int sgn(const R& r) { return (r>EPS) - (r<-EPS); }
namespace std {
	bool operator < (const P& a, const P& b) {
		return sgn(real(a-b)) ? real(a-b) < 0 : sgn(imag(a-b)) < 0;
	}
	bool operator == (const P& a, const P& b) {
		return sgn(real(a-b)) == 0 && sgn(imag(a-b)) == 0;
	}
}
inline R dot(const P& a, const P& b) {
  return real(a)*real(b) + imag(a)*imag(b);
}
inline R det(const P& a, const P& b) {
  return real(a)*imag(b) - imag(a)*real(b);
}
inline P vec(const L& l) {return l.second - l.first;}

// P,L,Sについて入力
inline istream& operator>>(istream& is, P& p) {
  R x, y;
  is >> x >> y;
  p = P(x, y);
  return is;
}
inline istream& operator>>(istream& is, L& l) {
  P a, b;
  is >> a >> b;
  l = L(a, b);
  return is;
}
inline istream& operator>>(istream& is, S& s) {
  P a, b;
  is >> a >> b;
  s = S(a, b);
  return is;
}

// 射影
P projection(const L &l, const P &p) {
  R t = dot(p-l.first, l.first-l.second) / norm(l.first-l.second);
  return l.first + t*(l.first-l.second);
}
// 反射
P reflection(const L &l, const P &p) {
  return p + (R)2 * (projection(l, p) - p);
}

// 線分abから見たcの位置
enum CCW{LEFT=1, RIGHT=2, BACK=4, FRONT=8, ON=16};
int ccw(P a, P b, P c) {
	P p = (c-a)/(b-a);
	if(sgn(imag(p)) > 0) return LEFT;
	if(sgn(imag(p)) < 0) return RIGHT;
	if(sgn(real(p)) < 0) return BACK;
	if(sgn(real(p)-1) > 0) return FRONT;
	return ON;
}

// 垂直,平行
inline bool vertical(L a, L b) {return sgn(dot(vec(a), vec(b))) == 0;}
inline bool parallel(L a, L b) {return sgn(det(vec(a), vec(b))) == 0;}
inline bool eal(L a, L b) {return vertical(a,b) && parallel(a,b);}

// 交差判定
template<bool strict=false> inline bool intersect(const L&l1, const L&l2) {
  if(strict) return sgn(det(vec(l1),vec(l2))) != 0;
  return sgn(det(vec(l1),vec(l2))) != 0 || l1 == l2;
}
template<bool strict=false> inline bool intersect(const L&l, const S&s) {
  if(strict) det(s.first, vec(l)) * det(s.second, vec(l)) < 0;
  return det(s.first, vec(l)) * det(s.second, vec(l)) <= 0;
}
template<bool strict=false> inline bool intersect(const S&s1, const S&s2) {
  int ccw1 = ccw(s1.first, s1.second, s2.first) | ccw(s1.first, s1.second, s2.second);
  int ccw2 = ccw(s2.first, s2.second, s1.first) | ccw(s2.first, s2.second, s1.second);
  if(strict) return (ccw1 & ccw2) == (LEFT | RIGHT);
  return (ccw1 & ccw2) == (LEFT | RIGHT) || ((ccw1 | ccw2) & ON);
}
template<bool strict=false> inline bool intersect(const S&s, const P&p) {
  return ccw(s.first, s.second, p) == ON;
}
template<bool strict=false> inline bool intersect(const L&l, const P&p) {
  return ccw(l.first, l.second, p) == ON ||
          ccw(l.first, l.second, p) == FRONT ||
          ccw(l.first, l.second, p) == BACK;
}

// 交点 交差判定を先にすること!!!
inline P crosspoint(const L& l1, const L& l2) {
  R ratio = det(vec(l2), l2.first-l1.first)/det(vec(l2),vec(l1));
  return l1.first + vec(l1)*ratio;
}

// 距離
R dist(const S& s, const P& p) {
  P q = projection(s, p);
  if(sgn(dot(s.second-s.first, p-s.first)) <= 0) q = s.first;
  if(sgn(dot(s.first-s.second, p-s.second)) <= 0) q = s.second;
  return abs(p-q);
}
R dist(const S& a, const S& b) {
  if(intersect(a, b)) return 0;
  return min({dist(a, b.first), dist(a, b.second), dist(b, a.first), dist(b, a.second)});
}

// 面積 頂点が反時計回りに並んでいること
R area(const G& pol) {
  R ret = 0.0;
  REP(i, pol.size()) ret += det(pol[i], pol[(i+1)%pol.size()]);
  return (ret/2.0);
}

signed main(void)
{
  cin.tie(0);
  ios::sync_with_stdio(false);

  int n;
  cin >> n;
  G pol(n);
  REP(i, n) cin >> pol[i];
  cout << fixed << setprecision(1) << area(pol) << endl;

  return 0;
}



Measured memory_bytes: 3300