#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int N;
double Result;
struct Point{
double x;
double y;
};
Point Array[100005];
void Read()
{
    f>>N;
    int i;
    for(i=1;i<=N;i++)
        f>>Array[i].x>>Array[i].y;
    Array[N+1].x=Array[1].x;
    Array[N+1].y=Array[1].y;
}
void Solve()
{
    int i;
    for(i=1;i<=N;i++)
        Result+=Array[i].x*Array[i+1].y-Array[i+1].x*Array[i].y;
    if(Result<0)
        Result*=-1;
    Result/=2;
    g<<fixed<<setprecision(6)<<Result<<"\n";
}
int main()
{
    Read();
    Solve();
    return 0;
}