#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef unsigned long long ull;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll myRand(ll B) {
    return (ull)rng() % B;
}

struct Point{
    ll x,y;
};

ll cross(Point a,Point b){
    return a.x*b.y-a.y*b.x;
}

ll area(const vector<Point> &g){
    ll res=0.0;
    for(int i=0;i<g.size();i++){
        res+=cross(g[i],g[(i+1)%g.size()]);
    }
    return res;
}

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n; cin >> n;
    vector<Point> v(n);
    for(auto &p:v){
        cin >> p.x >> p.y;
    }
    cout << area(v) << endl;
}