#include <fstream>

using namespace std;

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

struct cord
{
    double x;
    double y;
};

cord c[100001];
int n;
double s;

int main()
{
    in>>n;
    for(int i=1; i<=n; i++)
    {
        in>>c[i].x>>c[i].y;
    }
    for(int i=1; i<n; i++)
    {
        s+=c[i].x * c[i+1].y - c[i].y * c[i+1].x;
    }
    s += c[n].x * c[1].y - c[n].y * c[1].x;
    if(s<0) s*=-1;
    out<<s/2;
    return 0;
}