#include    <iomanip>
#include    <iostream>
#include    <fstream>

using namespace std;

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

const int NLIM = 100005;

double x[NLIM], y[NLIM], Aria;
int n;

void Read()
{
    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++)
        Aria += x[i] * y[i+1] - x[i+1] * y[i];

    fout << setprecision(20) << Aria / 2;
}

int main()
{
    Read();
    return 0;
}