#include <bits/stdc++.h>
#define MAXN 100005

using namespace std;

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

int n;
double sol;

struct bleh
{
    double x;
    double y;
};

bleh v[100005];

int main()
{
    in >> n;
    for (int i=1; i<=n; i++)
        in >> v[i].x >> v[i].y;
    v[n+1]=v[1];
    for (int i=1; i<=n; i++)
        sol+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    out << fixed << setprecision(6) << 0.5*abs(sol);
    return 0;
}