#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(long double x1,long double y1,long double x2,long double y2)
{
    return abs(x1*y2-y1*x2) / 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=1;i<=n-1;i++)
    {
        arie+=arie_triunghi(x[i],y[i],x[i+1],y[i+1]);
    }
    arie+=arie_triunghi(x[1],y[1],x[n],y[n]);
    fout << setprecision(6) << fixed << arie;
    return 0;
}