#include <bits/stdc++.h>
#define ld long double
#define x first
#define y second
using namespace std;
ifstream fi("aria.in");
ofstream fo("aria.out");

int n;
pair <ld, ld> P[100005];

int main()
{
    fi >> n;
    for (int i = 1; i <= n; i++)
    {
        fi >> P[i].x >> P[i].y;
    }
    P[n + 1] = P[1];

    ld rez = 0;
    for (int i = 1; i <= n; i++)
        rez += (P[i].x * P[i + 1].y - P[i].y * P[i + 1].x) / 2.00;

    fo << fixed << setprecision(5) << rez;

    return 0;
}