#include <iostream>
#include <fstream>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

const int NMax = 100005;
struct node{
    long long int x,y;
}v[NMax];

int main()
{
    int n;
    fin >> n;
    for(int i = 1; i <= n; i++){
        fin >> v[i].x >> v[i].y;
    }
    v[n + 1].x = v[1].x;
    v[n + 1].y = v[1].y;
    long long int ans = 0;
    for(int i = 1; i <= n; i++){
        ans = ans + v[i].x * v[i + 1].y - v[i].y * v[i + 1].x;
    }
    fout << ans / 2;
    return 0;
}