#include <cstdio>
#include <cmath>
#include <iostream>
#include <iomanip>

using namespace std;

#define MAX 100005

struct punct {
    long double x, y;
} V[MAX];

int N;

int main () {
    freopen ("aria.in", "r", stdin);
    freopen ("aria.out", "w", stdout);

    scanf ("%d", &N);
    for (int i = 0; i < N; ++i) {
        cin >> V[i].x >> V[i].y;
    }

    V[N] = V[0];

    long double answer = 0.0;

    for (int i = 0; i < N; ++i) {
        answer += (V[i].x * V[i + 1].y - V[i + 1].x * V[i].y);
    }

    cout << setprecision(6) << fixed << fabs(answer / 2.0) << '\n';

    return 0;
}