#include <bits/stdc++.h>

using namespace std;

const int NMAX = 100005;

struct Point {
    float x, y;
} V[NMAX];

int N;
float sum;

int main()
{
    ios_base::sync_with_stdio(NULL);
    cin.tie(NULL), cout.tie(NULL);

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

    cin >> N;

    for(int i = 0; i < N; ++i)
        cin >> V[i].x >> V[i].y;

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

    cout << setprecision(5) << fixed;
    cout << sum / 2.0;

    return 0;
}