#include <cstdio>
#include <vector>
#include <stack>
#include <algorithm>
#include <cmath>
#include <queue>
#include <set>
#include <iomanip>
#include <cstring>
#include <fstream>

#define NMax 100005
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

struct punct{
    double x;
    double y;
}v[NMax];

double ANS;
int n;

double arie(punct A, punct B, punct C){
    return ((A.x*B.y + C.x*A.y + B.x*C.y - C.x*B.y - A.x*C.y - B.x*A.y) / 2);
}
void citeste(){
    f >> n;
    for(int i = 1; i <= n; ++i)
        f >> v[i].x >> v[i].y;
}
void rezolva(){
  // ANS += arie(v[1],v[2],v[3]);
    for(int i = 3; i <= n; ++i){
        ANS += arie(v[1], v[i - 1], v[i]);
    }
}
void scrie(){
    g << fixed << setprecision(5) << ANS;
}
int main()
{
    citeste();
    rezolva();
    scrie();
    return 0;
}