#include <fstream>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n,x[100001],y[100001],i;
double aria;
int main()
{
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(i=1;i<=n;i++)
    {
        aria+=x[i]*y[i+1]-x[i+1]*y[i];
    }

    fout<<aria/2;

    return 0;
}