#include <iostream>
#include <fstream>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <iomanip>
#include <climits>
#include <stack>

using namespace std;

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

struct costi
{
    float x, y;
}v[100005];

int main()
{
    int n;
    float s = 0;
    fin >> n;
    for (int i = 1; i <= n; i++)
        fin >> v[i].x >> v[i].y;
    v[n + 1].x = v[1].x;
    v[n + 1].y = v[1].y;
    for (int i = 1; i <= n; i++)
        s += v[i].x * v[i + 1].y - v[i + 1].x * v[i].y;
    if (s < 0)
        s = -s;
    s = s / 2;
    fout << s << '\n';
    return 0;
}