#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vc;
typedef vector<vector<int>> matrix;
#define ft(n) for(ull i=1; i<=n; i++)
#define sp ' '
string file = "aria";
ifstream f(file + ".in");
ofstream g(file + ".out");

struct pct
{
    long double x, y;
};
vector<pct> v;
int n;
long double area;

int main()
{
    f >> n;
    v.resize(n + 5, { 0, 0 });
    f >> v[1].x >> v[1].y;
    v[n + 1] = v[1];
    for (int i = 2; i <= n+1; i++)
    {
        f >> v[i].x >> v[i].y;
        area += v[i-1].x * v[i].y - v[i].x * v[i-1].y; //x3, y3 = 0, 0
    }
    area /= 2;
    g << fixed << setprecision(6) << area;
    return 0;
}