#include <algorithm>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <string>
#include <utility>
#include <vector>

using namespace std;
#define rep(i, n) for (int i = 0; i < n; i++)
#define rep1(i, n) for (int i = 1; i < n + 1; i++)
#define all(A) A.begin(), A.end()
typedef long long ll;

double heron(pair<int,int> start,pair<int,int> target1,pair<int,int> target2){
  target1.first -= start.first;
  target1.second -= start.second;
  target2.first -= start.first;
  target2.second -= start.second;
  return abs(target1.first*target2.second-target2.first*target1.second);
}

int main() {
  int n;
  cin >> n;
  vector<pair<int,int>> xy(n);
  rep(i,n){
    int x,y;
    cin >> x >> y;
    xy[i] = make_pair(x,y);
  }
  int ans = 0;
  rep(i,n-2){
    ans += heron(xy[0],xy[i+1],xy[i+2]);
  }
  cout << ans << endl;
}