#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#include <time.h>
#include <stack>
#include <array>
#include <list>
#include <atcoder/all>
#define popcount __builtin_popcount
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef pair<int, int> P;

int main()
{
    int n; cin>>n;
    ll x[33], y[33];
    for(int i=0; i<n; i++){
        cin>>x[i]>>y[i];
    }
    x[n]=x[0], y[n]=y[0];
    ll ans=0;
    for(int i=0; i<n; i++){
        ans+=x[i]*y[i+1]-y[i]*x[i+1];
    }
    cout<<ans<<endl;
    return 0;
}