#include <bits/stdc++.h>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");
struct Punct {
    int x, y;
} v[100002];
int n, i, r;

static inline int Det(Punct p1, Punct p2, Punct p3) {
    return ((p2.x - p1.x) * (p3.y - p1.y) - (p3.x - p1.x) * (p2.y - p1.y));
}

int main() {
    fin >> n;
    for(i = 1; i <= n; i++) fin >> v[i].x >> v[i].y;
    v[0] = v[n];
    for(i = 0; i < n; i++) r += Det(v[0], v[i], v[i + 1]);
    fout << fixed << setprecision(5) << r / 2.0;

    return 0;
}