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

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

const int nmax = 100010;

double x[nmax],y[nmax],A=0;
int n,i;

int main()
{
    cin>>n;
    for(i=1;i<=n;i++) cin>>x[i]>>y[i];
    x[n+1]=x[1]; y[n+1]=y[1];
    for (i=1;i<=n;i++) A+=x[i]*y[i+1]-x[i+1]*y[i];
    cout<<setprecision(20)<<A/2;
    return 0;
}