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

using namespace std;

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

const int MAXN = 100010;

double X[MAXN], Y[MAXN];

inline double modul (const double &A)
{
    if (A < 0)
        return -A;

    return A;
}

int main()
{
    int N, i;
    double Area = 0;

    in >> N;
    for (i = 1; i <= N; i ++)
        in >> X[i] >> Y[i];

    X[N + 1] = X[1];
    Y[N + 1] = Y[1];

    for (i = 1; i <= N; i ++)
        Area += 0.5 * (X[i] * Y[i + 1] - X[i + 1] * Y[i]);

    out << fixed << setprecision (5) << modul (Area);

    return 0;
}