#include <fstream>
#include<iomanip>
#define FOR(i,a,b) for(int i = a; i <= b; i++)
#define NMAX 100005
using namespace std;

double a[NMAX],b[NMAX],aria;

ifstream fin("aria.in");
ofstream fout("aria.out");
int N;
int main()
{
    fin>>N;
    FOR(i,1,N)
    {
        fin>>a[i]>>b[i];
    }
    FOR(i,1,N)
    {
        aria+=a[i]*b[i+1]-a[i+1]*b[i];
    }
    aria+=a[N]*b[1]-a[1]*b[N];
    aria = (double) aria/2.0;
    fout<<setprecision(5)<<(double)aria<<'\n';
    return 0;
}