#include <bits/stdc++.h>
#define NMAX 100005
/// DACA SUNT SORTATE IN ORDINE TRIGONOMETRICA
/// P (0, 0)
/// det devine (x[i] * y[i + 1] - x[i + 1] * y[i]) / 2

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

int N;
long double S;
long double x[NMAX], y[NMAX];

int main()
{
    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] * y[i + 1] - x[i + 1] * y[i];
    S /= 2;
    fout << fixed << setprecision(5) << S << "\n";
    return 0;
}