#include <fstream>
#include <iomanip>
#include <cmath>
using namespace std;

const int Nmax = 100010;

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

typedef pair<double,double> Pair;

#define x first
#define y second

int N;
Pair P[Nmax];
double Out;

int main ()
{
    F>>N;
    for (int i=1;i<=N;++i)
        F>>P[i].x>>P[i].y;
    P[++N]=P[1];
    for (int i=1;i<N;++i)
        Out += (1LL*P[i].x*P[i+1].y) - (1LL*P[i+1].x*P[i].y);

    Out = fabs(Out) / 2.0;
    G<<fixed<<setprecision(6)<<Out<<'\n';
}