#include <bits/stdc++.h>
#include<iomanip>

using namespace std;

ifstream fin ("aria.in");
ofstream fout ("aria.out");
double x[100001], y[100001];
int n;

double solve()
{
    double s = 0;
    fin >> n;
    for(int i = 1; i <= n; i++)
    {
        fin >> x[i] >> y[i];
    }
    x[n+1] = x[1];
    y[n+1] = y[1];
    for(int i = 1; i <= n; i++)
    {
        s+= x[i+1] * y[i] - x[i]*y[i+1];
    }
    if(s<0) s=s*(-1);
    return s/2;
}

int main()
{
    fout << fixed << setprecision(5) << solve();
    return 0;
}