text: #include <bits/stdc++.h>
  // #include <atcoder/all>
  // using namespace atcoder;
  typedef long long ll;
  typedef long double ld;
  #define rep(i,n) for(ll i=0;i<(n);i++)
  #define repr(i,n) for(ll i=(n-1);i>=0;i--)
  #define all(x) x.begin(),x.end()
  #define br cout << '\n';
  using namespace std;
  const long long INF = 8e18;
  using Graph = vector<vector<ll>>;
  template<class T> inline bool chmin(T &a, T b) { if(a > b){ a = b; return true;} return false;}
  template<class T> inline bool chmax(T &a, T b) { if(a < b){ a = b; return true;} return false;}
  ll ceilll(ll a, ll b) {return (a + b-1) / b;} // if(a%b != 0) (a/b) + 1
  ll get_digit(ll a) {ll digit = 0; while(a != 0){a /= 10; digit++;} return digit;} // a != 0
  template<typename T> void vecdbg(vector<T>& v){ rep(i, v.size()){cerr << v[i] << ' ';} cerr << '\n';}
  template<typename T> void vecvecdbg(vector<vector<T>>& v){ rep(i, v.size()){rep(j, v[i].size()){cerr << v[i][j] << ' ';} cerr << '\n';}}
  ll POW(ll a, ll n){ ll res = 1; while(n > 0){ if(n & 1){ res = res * a; } a *= a; n >>= 1; } return res; }
  using P = pair<ll, ll>;
  const double PI = acos(-1);
  
  // 0 false, 1 true 
  // string number to int : -48 or - '0'
  // a to A : -32
  // ceil(a)  1.2->2.0
  // c++17	g++ -std=c++17 a.cpp
  // global vector -> 0 initialization
  // DONT FORGET TO INTIALIZE
  // The type of GRID is CHAR. DONT USE STRING
  // If the result in local and judge is different, USE CODETEST!!
  // (a * b)over flow?   if(a > INF / b){ /* overflow */}
  // for(auto& i: mp) &&&&&&&&&&&&&
  
  int main() {
      std::cout << std::fixed << std::setprecision(15);
      ll n; cin >> n;
      vector<ll> x(n);
      vector<ll> y(n);
      vector<ll> gaiseki(n);
      vector<ll> gaiseki_sum(n);
      rep(i, n){
          cin >> x[i] >> y[i];
      }
      ll area = 0;
      rep(i, n){
          area += (x[i]*y[(i+1) % n] - x[(i+1) % n]*y[i]);
          gaiseki[i] = (x[i]*y[(i+1) % n] - x[(i+1) % n]*y[i]);
          gaiseki_sum[i+1] = gaiseki_sum[i] + gaiseki[i];
      }
      vecdbg(gaiseki);
      cout << area << endl;
  
      ll ans = INF;
  
      rep(i, n){
          ll l = 2, r = n+1; // ok: l, ng: r
          ll midl, midr;
          while(r - l > 1){
              midl = (l*2 + r) / 3;
              midr = (l + r*2) / 3;
  
              
          }
      }
  
  
  
  
  
  
  
  }