#include <bits/stdc++.h>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct point
{
    int x,y;
};
vector<point> v;
int main()
{
    int n,x,y,ans=0;
    fin>>n;
    v.push_back({-1,-1});
    for(int i=1;i<=n;i++)
    {
        fin>>x>>y;
        v.push_back({x,y});
    }
    for(int i=1;i<=n;i++)
    {
        int j=i+1;
        if(i==n) j=1;
        int arr=(v[i].x*v[j].y-v[j].x*v[i].y)/2;
        ans+=arr;
    }
    fout<<ans;
    return 0;
}