#include <fstream>
using namespace std;

const int MAXN = 100005;

int n;
int x[MAXN], y[MAXN];

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

inline int modul(int x)
{
    return (x < 0)?-x:x;
}

void citire()
{
    in >> n;
    for (int i = 1;i <= n;++i)
        in >> x[i] >> y[i];
}

double arie()
{
    double sol = 0;
    x[n + 1] = x[1];
    y[n + 1] = y[1];
    for (int i = 1;i <= n;++i)
        sol += x[i] * y[i + 1] - x[i + 1] * y[i];
    return modul(sol / 2);
}

int main()
{
    citire();
    out << arie() << '\n';
    return 0;
}