#include<bits/stdc++.h>
#define rep(i, n) for(int i=0;i<(n);++i)
#define per(i, n) for(int i=(n)-1;i>=0;--i)
#define repa(i, n) for(int i=1;i<(n);++i)
#define foreach(i, n) for(auto &i:(n))
#define pii pair<int, int>
#define pll pair<long long, long long>
#define all(x) (x).begin(), (x).end()
#define bit(x) (1ll << (x))
using namespace std;
using ll = long long;
const ll MOD = (ll)1e9+7;
//const ll MOD = 998244353;
//const ll MOD = 924844033;
const int INF = (ll)1e9+7;
const ll INFLL = (ll)1e18;
const double EPS = 1e-5;
using namespace std;
template<class t>
using vvector = vector<vector<t>>;
template<class t>
using vvvector = vector<vector<vector<t>>>;
template<class t>
using priority_queuer = priority_queue<t, vector<t>, greater<t>>;
template<class t, class u> bool chmax(t &a, u b){if(a<b){a=b;return true;}return false;}
template<class t, class u> bool chmin(t &a, u b){if(a>b){a=b;return true;}return false;}

#ifdef DEBUG
#define OUTPUT(x) (output(x), outendl())
#else
#define OUTPUT(x) (void)0
#endif

ll modpow(ll x, ll b){
	ll res = 1;
	while(b){
		if(b&1)res = res * x % MOD;
		x = x * x % MOD;
		b>>=1;
	}
	return res;
}

ll modinv(ll x){
	return modpow(x, MOD-2);
}

bool was_output = false;
template<class t>
void output(t a){
	if(was_output)cout << " ";
	cout << a;
	was_output = true;
}
void outendl(){
	was_output = false;
	cout << endl;
}

//below here


class Vector2{
public:
  double x;
  double y;
  Vector2():x(0), y(0){}
  Vector2(double a, double b):x(a), y(b){}
  Vector2(const Vector2 &o):x(o.x), y(o.y){}

  Vector2& operator+=(Vector2 o){x+=o.x;y+=o.y;return *this;}
  Vector2& operator-=(Vector2 o){x-=o.x;y-=o.y;return *this; }
  Vector2& operator*=(double o){x*=o;y*=o;return *this;}
  Vector2& operator/=(double o){x/=o;y/=o;return *this;}

  Vector2 operator+(Vector2 o){return Vector2(*this)+=o;}
  Vector2 operator-(Vector2 o){return Vector2(*this)-=o;}
  Vector2 operator-(){return Vector2(-x,-y);}
  Vector2 operator*(double o){return Vector2(*this)*=o;}
  Vector2 operator/(double o){return Vector2(*this)/=o;}

  double abs(){
    return sqrt(mag());
  }

  double mag(){
    return x*x+y*y;
  }

  Vector2 normal(){
    return (*this) / abs();
  }

  Vector2 rotate_90(){
    return Vector2(-y,x);
  }

  static double dot(Vector2 a,Vector2 b){
    return a.x * b.x + a.y * b.y;
  }

  static double cross(Vector2 a,Vector2 b){
    return a.x * b.y - a.y * b.x;
  }
};


double dot(Vector2 x, Vector2 y){
  return Vector2::dot(x,y);
}

double cross(Vector2 x, Vector2 y){
  return Vector2::cross(x,y);
}

void output(Vector2 x){
  cout << x.x << " " << x.y << endl;
}

class Polygon{
public:
  vector<Vector2> p;
  Polygon(vector<Vector2> p_):p(p_){}
  Polygon(){}

  void add_point(Vector2 x){
    p.emplace_back(x);
  }

  double cul_area(){
    double res = 0;
    for(int i=1;i<p.size()-1;++i){
      res += Vector2::cross(p[i]-p[0],p[i+1]-p[0]);
    }
    return res * 0.5;
  }
};

Vector2 input_vector2(){
	double x,y;
	cin >> x >> y;
	return Vector2(x,y);
}

double func(){
  int n;
  cin >> n;
  vector<Vector2> ins(n);
  foreach(i,ins)i = input_vector2();
  return Polygon(ins).cul_area();
}

int main(){
  printf("%.1lf\n",func());
}



Measured memory_bytes: 3300