#include <bits/stdc++.h>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

const int nmax=1e5+4;
int n;
long double x[nmax],y[nmax];
long double area;

int main()
{
    fin>>n;
    for(int i=1; i<=n; i++)
    {
        fin>>x[i]>>y[i];
        area+=(x[i-1]*y[i]-x[i]*y[i-1])/2;
    }
    area+=(x[n]*y[1]-x[1]*y[n])/2;
    fout<<fixed<<setprecision(10)<<area;
    return 0;
}