/* -*- coding: utf-8 -*-
 *
 * 1447.cc:  No.1447 Greedy MtSaka - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 30;

/* typedef */

typedef long long ll;

/* global variables */

int xs[MAX_N], ys[MAX_N];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%d%d", xs + i, ys + i);

  ll sum = 0;
  int x0 = xs[0], y0 = ys[0];

  for (int i = 1; i + 1 < n; i++) {
    int dx0 = xs[i] - x0, dy0 = ys[i] - y0;
    int dx1 = xs[i + 1] - x0, dy1 = ys[i + 1] - y0;

    sum += abs((ll)dx0 * dy1 - (ll)dy0 * dx1);
  }

  printf("%lld\n", sum);
  return 0;
}