#include <bits/stdc++.h>

const int MAX_N = 100000;

using namespace std;

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

int n, x[MAX_N + 5], y[MAX_N + 5];
double s;

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 += 0.5 * (x[i] * y[i + 1] - y[i] * x[i + 1]);
    fout << fixed << setprecision(5) << s << '\n';

    return 0;
}