#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
long long n,x[100001],y[100001],i;
long 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];
    }

    if(aria<0) aria=-aria;
    aria/=(long double)2;

    fout<<setprecision(5)<<fixed<<aria;

    return 0;
}