#include <bits/stdc++.h>

using namespace std;

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

const int NMAX = 100005;
long double x[NMAX],y[NMAX];

long double arie_triunghi(int x1,int y1,int x2,int y2,int x3,int y3)
{
    return abs(x2*y3 - x1*y3 - x2*y1 - x3*y2 + x3*y1 + x1*y2); /// / 2;
}

int main()
{
    int n;
    fin >> n;
    for(int i=1;i<=n;i++)
    {
        fin >> x[i] >> y[i];
    }
    long double arie=0;
    for(int i=2;i<=n-1;i++)
    {
        arie+=arie_triunghi(x[1],y[1],x[i],y[i],x[i+1],y[i+1]);
    }
    arie=1LL*(double)arie*0.5;
    fout << setprecision(6) << fixed << arie;
    return 0;
}