#line 2 "/home/defineprogram/Desktop/Library/template/template.cpp"
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define rep(i, n) for (int i = 0; i < n; i++)
#define REP(i, n) for (int i = 1; i < n; i++)
#define rev(i, n) for (int i = n - 1; i >= 0; i--)
#define REV(i, n) for (int i = n - 1; i > 0; i--)
#define all(v) v.begin(), v.end()
#define PL pair<ll, ll>
#define PI pair<int,int>
#define len(s) (int)s.size()

template <class T, class U>
inline bool chmin(T &a, U b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template <class T, class U>
inline bool chmax(T &a, U b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
constexpr ll inf = 3e18;
#line 2 "main.cpp"

ll f(ll a,ll b,ll c,ll d,ll e,ll f){
    return a*d+c*f+e*b-b*c-d*e-f*a;
}
ll X[35],Y[35];
int main() {
    cin.tie(0); ios::sync_with_stdio(false);
    int N;cin>>N;
    rep(i,N)cin>>X[i]>>Y[i];
    ll res=0;
    REP(i,N-1){
        res+=f(X[0],Y[0],X[i],Y[i],X[i+1],Y[i+1]);
    }
    cout<<res<<"\n";
}