#include <fstream>
#include <iomanip>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
int n,i;
double aria;
struct punct
{
    long long x;
    long long y;
};
punct varf[100005];
long long arie(punct A,punct B)
{
    long long cross;
    cross=A.x*B.y-A.y*B.x;
    return cross;
}
int main()
{
    cin>>n;
    for(i=1;i<=n;i++)
        cin>>varf[i].x>>varf[i].y;
    varf[n+1]=varf[1];
    for(i=1;i<=n;i++)
        aria+=arie(varf[i],varf[i+1]);
    aria/=2.0;
    cout<<fixed<<setprecision(5)<<aria;
    return 0;
}