#include <iostream>
#include <fstream>
#include <iomanip>
#define DIM 100005
#define pr pair<long double, long double>
#define x first
#define y second

using namespace std;

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

int n;

long double A, B, doi = 2;

pr p[DIM], pctar;


int main() {
    in>>n;
    for(int i = 1; i <= n; ++ i)
        in>>p[i].x>>p[i].y;
    pctar = make_pair(0, 0);
    p[n + 1] = p[1];
    p[n + 2] = p[2];
    for(int i = 1; i <= n / 2; ++ i){
        A = A + ((p[i].x * p[i + 1].y) - (p[i + 1].x * p[i].y)) / doi;
    }
    for(int i = n / 2 + 1; i <= n; ++ i){
        B = B + ((p[i].x * p[i + 1].y) - (p[i + 1].x * p[i].y)) / doi;
    }
  //  A = A / (long double)(2.0);
 //   B = B / (long double)(2.0);
    long double S = A + B;
    out<<fixed<<setprecision(5)<<(long double)(S);
    return 0;
}