#include <fstream>
using namespace std;
struct punct
{
    int x,y;
}v[100002];
int main()
{
    int n,i;
    long long s;
    ifstream in("aria.in");
    ofstream out("aria.out");
    in>>n;
    for(i=1;i<=n;i++)
        in>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    s=0;
    for(i=1;i<=n;i++)
    {
        s=s+(v[i].x*v[i+1].y)-(v[i+1].x*v[i].y);
    }
    out<<s/2.0;
    return 0;
}