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

long double S=0,x[3],y[3],a,b;
int N;

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

    fin>>a>>b;
    x[1]=a;
    y[1]=b;

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

        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);
    S = S*0.5;

    fout<<fixed<<setprecision(7)<<S;

    return 0;
}