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

double x[2],y[2],a,b;
long double S=0;
int N;

int main()
{
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    fin>>N;

    for(int i=1;i<=N;++i)
    {
        fin>>x[0]>>y[0];
        if(i==1)
        {
            a = x[0];
            b = y[0];
        }

        else
        {
            S = S + x[1]*y[0] - x[0]*y[1];
        }

        x[1] = x[0];
        y[1] = y[0];

    }

    S = S + b*x[1] - a*y[1];

    S = abs(S/2);

    fout<<fixed<<setprecision(6)<<S;

    return 0;
}