#include<fstream>
using namespace std;
int n, i, s;
struct cub{
    int x;
    int y;
};
cub zero, v[100003];
double det(cub a, cub b, cub c){
    return ((b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y))/2.0;
}
ifstream in("aria.in");
ofstream out("aria.out");
int main(){
    in>>n;
    for(i=1; i<=n; i++)
        in>>v[i].x>>v[i].y;
    v[0]=v[n];
    for(i=0; i<n; i++)
        s+=det(zero, v[i], v[i+1]);
    out<<s;
return 0;
}