#include <bits/stdc++.h>

using namespace std;

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

const int DIM = 1e5 + 17;

long double x[DIM];
long double y[DIM];

main()
{
    int n;
    in >> n;

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

    x[n + 1] = x[1];
    y[n + 1] = y[1];

    long double ans = 0;

    for(int i = 1; i <= n; i++)
        ans += x[i] * y[i + 1] - x[i + 1] * y[i];

    ans /= 2;

    if(ans < 0)
        ans = -ans;

    out << fixed << setprecision(7) << ans;
}