#include <fstream>
#include <stdlib.h>

using namespace std;

ifstream cin("aria.in");
ofstream cout("aria.out");

int n;long long a,b,a1,b1,a2,b2;
long long aria;

int main()
{
    cin>>n>>a>>b>>a1>>b1;
    a1-=a;b1-=b;
    for (int i=3;i<=n;i++)
    {
        cin>>a2>>b2;
        a2-=a;b2-=b;
        aria+=a1*b2-a2*b1;
        a1=a2;b1=b2;
    }
    cout<<abs(aria)/2<<'\n';
    cin.close();cout.close();
    return 0;
}