#include<iostream>
#include<fstream>
using namespace std;
int n;
struct punct{

    long x;
    long y;

};
punct a[100010];
long long arie;
int main(){


    ifstream f("aria.in",fstream::in);
    ofstream g("aria.out",fstream::out);
    f>>n;

    for(int i=0;i<n;i++)
        f>>a[i].x>>a[i].y;

    for(int i=0;i<n-1;i++)
        arie=arie+(a[i].x*a[i+1].y-a[i+1].x*a[i].y);

    arie=arie+a[n-1].x*a[0].y-a[0].x*a[n-1].y;
    arie=arie/2;
    g<<arie;

}